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Re: [Qemu-devel] [PATCH QEMU-XEN v4 6/9] xen: Switch uses of xc_map_fore


From: Stefano Stabellini
Subject: Re: [Qemu-devel] [PATCH QEMU-XEN v4 6/9] xen: Switch uses of xc_map_foreign_bulk to use libxenforeignmemory API.
Date: Fri, 23 Oct 2015 12:06:51 +0100
User-agent: Alpine 2.02 (DEB 1266 2009-07-14)

On Wed, 21 Oct 2015, Ian Campbell wrote:
> In Xen 4.7 we are refactoring parts libxenctrl into a number of
> separate libraries which will provide backward and forward API and ABI
> compatiblity.
> 
> One such library will be libxenforeignmemory which provides access to
> privileged foreign mappings and which will provide an interface
> equivalent to xc_map_foreign_bulk.
> 
> In preparation for adding support for libxenforeignmemory add support
> to the <=4.0 and <=4.6 compat code in xen_common.h to allow us to
> switch to using the new API. These shims will disappear for versions
> of Xen which include libxenforeignmemory.
> 
> Since libxenforeignmemory will have its own handle type but for <= 4.6
> the functionality is provided by using a libxenctrl handle we
> introduce a new global xen_fmem alongside the existing xen_xc. In fact
> we make xen_fmem a pointer to the existing xen_xc, which then works
> correctly with both <=4.0 (xc handle is an int) and <=4.6 (xc handle
> is a pointer). In the latter case xen_fmem is actually a double
> indirect pointer, but it all falls out in the wash.
> 
> Unlike libxenctrl libxenforeignmemory has an explicit unmap function,
> rather than just specifying that munmap should be used, so the unmap
> paths are updated to use xenforeignmemory_unmap, which is a shim for
> munmap on these versions of xen. The mappings in xen-hvm.c do not
> appear to be unmapped (which makes sense for a qemu-dm process)
> 
> In fb_disconnect this results in a change from simply mmap over the
> existing mapping (with an implciit munmap) to expliclty unmapping with
                            ^ implicit

> xenforeignmemory_unmap and then mapping the required anonymous memory
> in the same hole. I don't think this is a problem since any other
> thread which was racily touching this region would already be running
> the risk of hitting the mapping halfway through the call. If this is
> thought to be a problem then we could consider adding an extra API to
> the libxenforeignmemory interface to replace a foreign mapping with
> anonymous shared memory, but I'd prefer not to.
> 
> Build tested with 4.0 and 4.5.
> 
> Signed-off-by: Ian Campbell <address@hidden>

Reviewed-by: Stefano Stabellini <address@hidden>


> I noticed in xen_console.c that the decision to use a foreign
> privileged memory mapping vs a grant dev is made using different
> variables in con_initialise vs con_disconnect. The former uses
> xendev->dev while the latter uses xendev->gnttabdev. Is this a latent
> bug?
> 
> v4: Rebase onto "xen_console: correctly cleanup primary console on
>     teardown."
> 
>     xenforeignmemory_unmap takes pages not bytes
> 
>     Compat wrapper for xenforeignmemory_open instead of ifdef in code.
> 
>     Run check patch and fix most issues. I did not fix:
> 
> ERROR: do not initialise globals to 0 or NULL
> +xenforeignmemory_handle *xen_fmem = NULL;
> 
> => This is consistent with all of the existing declarations.
> 
> ERROR: need consistent spacing around '*' (ctx:WxV)
> +typedef xc_interface *xenforeignmemory_handle;
> 
> => I think this is a false +ve since this is a pointer "*" not a multiple "*".
> ---
>  hw/char/xen_console.c        |  8 ++++----
>  hw/display/xenfb.c           | 15 ++++++++-------
>  hw/xen/xen_backend.c         |  3 ++-
>  include/hw/xen/xen_backend.h |  1 +
>  include/hw/xen/xen_common.h  | 12 ++++++++++++
>  xen-common.c                 |  6 ++++++
>  xen-hvm.c                    | 18 +++++++++---------
>  xen-mapcache.c               |  6 +++---
>  8 files changed, 45 insertions(+), 24 deletions(-)
> 
> diff --git a/hw/char/xen_console.c b/hw/char/xen_console.c
> index 11c6472..24f3a40 100644
> --- a/hw/char/xen_console.c
> +++ b/hw/char/xen_console.c
> @@ -230,9 +230,9 @@ static int con_initialise(struct XenDevice *xendev)
>  
>      if (!xendev->dev) {
>          xen_pfn_t mfn = con->ring_ref;
> -        con->sring = xc_map_foreign_bulk(xen_xc, con->xendev.dom,
> -                                         PROT_READ|PROT_WRITE,
> -                                         &mfn, &err, 1);
> +        con->sring = xenforeignmemory_map(xen_fmem, con->xendev.dom,
> +                                          PROT_READ|PROT_WRITE,
> +                                          &mfn, &err, 1);
>      } else {
>          con->sring = xengnttab_map_grant_ref(xendev->gnttabdev, 
> con->xendev.dom,
>                                               con->ring_ref,
> @@ -274,7 +274,7 @@ static void con_disconnect(struct XenDevice *xendev)
>  
>      if (con->sring) {
>          if (!xendev->dev) {
> -            munmap(con->sring, XC_PAGE_SIZE);
> +            xenforeignmemory_unmap(xen_fmem, con->sring, 1);
>          } else {
>              xengnttab_munmap(xendev->gnttabdev, con->sring, 1);
>          }
> diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c
> index b0ac1e6..a5ddb60 100644
> --- a/hw/display/xenfb.c
> +++ b/hw/display/xenfb.c
> @@ -103,8 +103,8 @@ static int common_bind(struct common *c)
>      if (xenstore_read_fe_int(&c->xendev, "event-channel", 
> &c->xendev.remote_port) == -1)
>       return -1;
>  
> -    c->page = xc_map_foreign_bulk(xen_xc, c->xendev.dom,
> -                                  PROT_READ | PROT_WRITE, &mfn, &err, 1);
> +    c->page = xenforeignmemory_map(xen_fmem, c->xendev.dom,
> +                                   PROT_READ | PROT_WRITE, &mfn, &err, 1);
>      if (c->page == NULL)
>       return -1;
>  
> @@ -119,7 +119,7 @@ static void common_unbind(struct common *c)
>  {
>      xen_be_unbind_evtchn(&c->xendev);
>      if (c->page) {
> -     munmap(c->page, XC_PAGE_SIZE);
> +        xenforeignmemory_unmap(xen_fmem, c->page, 1);
>       c->page = NULL;
>      }
>  }
> @@ -491,14 +491,14 @@ static int xenfb_map_fb(struct XenFB *xenfb)
>      errs = g_malloc0(sizeof(int) * n_fbdirs);
>  
>      xenfb_copy_mfns(mode, n_fbdirs, pgmfns, pd);
> -    map = xc_map_foreign_bulk(xen_xc, xenfb->c.xendev.dom,
> -                              PROT_READ, pgmfns, errs, n_fbdirs);
> +    map = xenforeignmemory_map(xen_fmem, xenfb->c.xendev.dom,
> +                               PROT_READ, pgmfns, errs, n_fbdirs);
>      if (map == NULL)
>       goto out;
>      xenfb_copy_mfns(mode, xenfb->fbpages, fbmfns, map);
> -    munmap(map, n_fbdirs * XC_PAGE_SIZE);
> +    xenforeignmemory_unmap(xen_fmem, map, n_fbdirs);
>  
> -    xenfb->pixels = xc_map_foreign_bulk(xen_xc, xenfb->c.xendev.dom,
> +    xenfb->pixels = xenforeignmemory_map(xen_fmem, xenfb->c.xendev.dom,
>              PROT_READ, fbmfns, errs, xenfb->fbpages);
>      if (xenfb->pixels == NULL)
>       goto out;
> @@ -907,6 +907,7 @@ static void fb_disconnect(struct XenDevice *xendev)
>       *   Replacing the framebuffer with anonymous shared memory
>       *   instead.  This releases the guest pages and keeps qemu happy.
>       */
> +    xenforeignmemory_unmap(xen_fmem, fb->pixels, fb->fbpages);
>      fb->pixels = mmap(fb->pixels, fb->fbpages * XC_PAGE_SIZE,
>                        PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANON,
>                        -1, 0);
> diff --git a/hw/xen/xen_backend.c b/hw/xen/xen_backend.c
> index f988b95..30ffcb4 100644
> --- a/hw/xen/xen_backend.c
> +++ b/hw/xen/xen_backend.c
> @@ -45,6 +45,7 @@
>  
>  /* public */
>  XenXC xen_xc = XC_HANDLER_INITIAL_VALUE;
> +xenforeignmemory_handle *xen_fmem = NULL;
>  struct xs_handle *xenstore = NULL;
>  const char *xen_protocol;
>  
> @@ -719,7 +720,7 @@ int xen_be_init(void)
>          goto err;
>      }
>  
> -    if (xen_xc == XC_HANDLER_INITIAL_VALUE) {
> +    if (xen_xc == XC_HANDLER_INITIAL_VALUE || xen_fmem == NULL) {
>          /* Check if xen_init() have been called */
>          goto err;
>      }
> diff --git a/include/hw/xen/xen_backend.h b/include/hw/xen/xen_backend.h
> index 8e8857b..e0d52ee 100644
> --- a/include/hw/xen/xen_backend.h
> +++ b/include/hw/xen/xen_backend.h
> @@ -57,6 +57,7 @@ struct XenDevice {
>  
>  /* variables */
>  extern XenXC xen_xc;
> +extern xenforeignmemory_handle *xen_fmem;
>  extern struct xs_handle *xenstore;
>  extern const char *xen_protocol;
>  
> diff --git a/include/hw/xen/xen_common.h b/include/hw/xen/xen_common.h
> index db62df4..2a5f27a 100644
> --- a/include/hw/xen/xen_common.h
> +++ b/include/hw/xen/xen_common.h
> @@ -41,6 +41,7 @@ static inline void *xc_map_foreign_bulk(int xc_handle, 
> uint32_t dom, int prot,
>  typedef int XenXC;
>  typedef int xenevtchn_handle;
>  typedef int xengnttab_handle;
> +typedef int xenforeignmemory_handle;
>  
>  #  define XC_INTERFACE_FMT "%i"
>  #  define XC_HANDLER_INITIAL_VALUE    -1
> @@ -104,6 +105,11 @@ static inline XenXC xen_xc_interface_open(void *logger, 
> void *dombuild_logger,
>      return xc_interface_open();
>  }
>  
> +#define xenforeignmemory_open(l, f) &xen_xc
> +#define xenforeignmemory_map(h, d, p, a, e, n) \
> +    xc_map_foreign_bulk(*h, d, p, a, e, n)
> +#define xenforeignmemory_unmap(h, p, s) munmap(p, s * XC_PAGE_SIZE)
> +
>  static inline int xc_fd(int xen_xc)
>  {
>      return xen_xc;
> @@ -149,6 +155,7 @@ static inline void xs_close(struct xs_handle *xsh)
>  #else
>  
>  typedef xc_interface *XenXC;
> +typedef xc_interface *xenforeignmemory_handle;
>  typedef xc_evtchn xenevtchn_handle;
>  typedef xc_gnttab xengnttab_handle;
>  
> @@ -178,6 +185,11 @@ static inline XenXC xen_xc_interface_open(void *logger, 
> void *dombuild_logger,
>      return xc_interface_open(logger, dombuild_logger, open_flags);
>  }
>  
> +#define xenforeignmemory_open(l, f) &xen_xc
> +#define xenforeignmemory_map(h, d, p, a, e, n) \
> +    xc_map_foreign_bulk(*h, d, p, a, e, n)
> +#define xenforeignmemory_unmap(h, p, s) munmap(p, s * XC_PAGE_SIZE)
> +
>  /* FIXME There is now way to have the xen fd */
>  static inline int xc_fd(xc_interface *xen_xc)
>  {
> diff --git a/xen-common.c b/xen-common.c
> index 56359ca..dd8619b 100644
> --- a/xen-common.c
> +++ b/xen-common.c
> @@ -117,6 +117,12 @@ static int xen_init(MachineState *ms)
>          xen_be_printf(NULL, 0, "can't open xen interface\n");
>          return -1;
>      }
> +    xen_fmem = xenforeignmemory_open(0, 0);
> +    if (xen_fmem == NULL) {
> +        xen_be_printf(NULL, 0, "can't open xen fmem interface\n");
> +        xc_interface_close(xen_xc);
> +        return -1;
> +    }
>      qemu_add_vm_change_state_handler(xen_change_state_handler, NULL);
>  
>      return 0;
> diff --git a/xen-hvm.c b/xen-hvm.c
> index 0c84e30..8fe7397 100644
> --- a/xen-hvm.c
> +++ b/xen-hvm.c
> @@ -1214,9 +1214,9 @@ int xen_hvm_init(ram_addr_t *below_4g_mem_size, 
> ram_addr_t *above_4g_mem_size,
>      DPRINTF("buffered io page at pfn %lx\n", bufioreq_pfn);
>      DPRINTF("buffered io evtchn is %x\n", bufioreq_evtchn);
>  
> -    state->shared_page = xc_map_foreign_bulk(xen_xc, xen_domid,
> -                                             PROT_READ|PROT_WRITE,
> -                                             &ioreq_pfn, &map_err, 1);
> +    state->shared_page = xenforeignmemory_map(xen_fmem, xen_domid,
> +                                              PROT_READ|PROT_WRITE,
> +                                              &ioreq_pfn, &map_err, 1);
>      if (state->shared_page == NULL) {
>          hw_error(
>              "map shared IO page returned error %d(%d) handle=" 
> XC_INTERFACE_FMT,
> @@ -1227,8 +1227,8 @@ int xen_hvm_init(ram_addr_t *below_4g_mem_size, 
> ram_addr_t *above_4g_mem_size,
>      if (!rc) {
>          DPRINTF("shared vmport page at pfn %lx\n", ioreq_pfn);
>          state->shared_vmport_page =
> -            xc_map_foreign_bulk(xen_xc, xen_domid, PROT_READ|PROT_WRITE,
> -                                &ioreq_pfn, &map_err, 1);
> +            xenforeignmemory_map(xen_fmem, xen_domid, PROT_READ|PROT_WRITE,
> +                                 &ioreq_pfn, &map_err, 1);
>          if (state->shared_vmport_page == NULL) {
>              hw_error("map shared vmport IO page returned error %d (%d) 
> handle="
>                       XC_INTERFACE_FMT, errno, map_err, xen_xc);
> @@ -1237,10 +1237,10 @@ int xen_hvm_init(ram_addr_t *below_4g_mem_size, 
> ram_addr_t *above_4g_mem_size,
>          hw_error("get vmport regs pfn returned error %d, rc=%d", errno, rc);
>      }
>  
> -    state->buffered_io_page = xc_map_foreign_bulk(xen_xc, xen_domid,
> -                                                  PROT_READ|PROT_WRITE,
> -                                                  &bufioreq_pfn,
> -                                                  &map_err, 1);
> +    state->buffered_io_page = xenforeignmemory_map(xen_fmem, xen_domid,
> +                                                   PROT_READ|PROT_WRITE,
> +                                                   &bufioreq_pfn,
> +                                                   &map_err, 1);
>      if (state->buffered_io_page == NULL) {
>          hw_error("map buffered IO page returned error %d (%d)", errno, 
> map_err);
>      }
> diff --git a/xen-mapcache.c b/xen-mapcache.c
> index 66da1a6..7d4953c 100644
> --- a/xen-mapcache.c
> +++ b/xen-mapcache.c
> @@ -169,10 +169,10 @@ static void xen_remap_bucket(MapCacheEntry *entry,
>          pfns[i] = (address_index << (MCACHE_BUCKET_SHIFT-XC_PAGE_SHIFT)) + i;
>      }
>  
> -    vaddr_base = xc_map_foreign_bulk(xen_xc, xen_domid, PROT_READ|PROT_WRITE,
> -                                     pfns, err, nb_pfn);
> +    vaddr_base = xenforeignmemory_map(xen_fmem, xen_domid, 
> PROT_READ|PROT_WRITE,
> +                                      pfns, err, nb_pfn);
>      if (vaddr_base == NULL) {
> -        perror("xc_map_foreign_bulk");
> +        perror("xenforeignmemory_map");
>          exit(-1);
>      }
>  
> -- 
> 2.1.4
> 



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