Re: [Qemu-devel] [RFC PATCH] vfio: VFIO Driver core framework

[Christian Benvenuti (benve)]

> -----Original Message-----
> From: Alex Williamson [mailto:address@hidden
> Sent: Friday, November 11, 2011 10:04 AM
> To: Christian Benvenuti (benve)
> Cc: address@hidden; address@hidden; address@hidden;
> address@hidden; address@hidden; address@hidden; Aaron Fabbri
> (aafabbri); address@hidden; address@hidden; address@hidden;
> address@hidden; address@hidden; address@hidden;
> address@hidden; address@hidden
> Subject: RE: [RFC PATCH] vfio: VFIO Driver core framework
> 
> On Wed, 2011-11-09 at 18:57 -0600, Christian Benvenuti (benve) wrote:
> > Here are few minor comments on vfio_iommu.c ...
> 
> Sorry, I've been poking sticks at trying to figure out a clean way to
> solve the force vfio driver attach problem.

Attach o detach?

> > > diff --git a/drivers/vfio/vfio_iommu.c b/drivers/vfio/vfio_iommu.c
> > > new file mode 100644
> > > index 0000000..029dae3
> > > --- /dev/null
> > > +++ b/drivers/vfio/vfio_iommu.c
> <snip>
> > > +
> > > +#include "vfio_private.h"
> >
> > Doesn't the 'dma_'  prefix belong to the generic DMA code?
> 
> Sure, we could these more vfio-centric.

Like vfio_dma_map_page?

> 
> > > +struct dma_map_page {
> > > + struct list_head        list;
> > > + dma_addr_t              daddr;
> > > + unsigned long           vaddr;
> > > + int                     npage;
> > > + int                     rdwr;
> > > +};
> > > +
> > > +/*
> > > + * This code handles mapping and unmapping of user data buffers
> > > + * into DMA'ble space using the IOMMU
> > > + */
> > > +
> > > +#define NPAGE_TO_SIZE(npage)     ((size_t)(npage) << PAGE_SHIFT)
> > > +
> > > +struct vwork {
> > > + struct mm_struct        *mm;
> > > + int                     npage;
> > > + struct work_struct      work;
> > > +};
> > > +
> > > +/* delayed decrement for locked_vm */
> > > +static void vfio_lock_acct_bg(struct work_struct *work)
> > > +{
> > > + struct vwork *vwork = container_of(work, struct vwork, work);
> > > + struct mm_struct *mm;
> > > +
> > > + mm = vwork->mm;
> > > + down_write(&mm->mmap_sem);
> > > + mm->locked_vm += vwork->npage;
> > > + up_write(&mm->mmap_sem);
> > > + mmput(mm);              /* unref mm */
> > > + kfree(vwork);
> > > +}
> > > +
> > > +static void vfio_lock_acct(int npage)
> > > +{
> > > + struct vwork *vwork;
> > > + struct mm_struct *mm;
> > > +
> > > + if (!current->mm) {
> > > +         /* process exited */
> > > +         return;
> > > + }
> > > + if (down_write_trylock(&current->mm->mmap_sem)) {
> > > +         current->mm->locked_vm += npage;
> > > +         up_write(&current->mm->mmap_sem);
> > > +         return;
> > > + }
> > > + /*
> > > +  * Couldn't get mmap_sem lock, so must setup to decrement
> >                                                       ^^^^^^^^^
> >
> > Increment?
> 
> Yep
> 
> <snip>
> > > +int vfio_remove_dma_overlap(struct vfio_iommu *iommu, dma_addr_t
> > > start,
> > > +                     size_t size, struct dma_map_page *mlp)
> > > +{
> > > + struct dma_map_page *split;
> > > + int npage_lo, npage_hi;
> > > +
> > > + /* Existing dma region is completely covered, unmap all */
> >
> > This works. However, given how vfio_dma_map_dm implements the merging
> > logic, I think it is impossible to have
> >
> >     (start < mlp->daddr &&
> >      start + size > mlp->daddr + NPAGE_TO_SIZE(mlp->npage))
> 
> It's quite possible.  This allows userspace to create a sparse mapping,
> then blow it all away with a single unmap from 0 to ~0.

I would prefer the user to use exact ranges in the unmap operations
because it would make it easier to detect bugs/leaks in the map/unmap
logic used by the callers.
My assumptions are that:

- the user always keeps track of the mappings

- the user either unmaps one specific mapping or 'all of them'.
  The 'all of them' case would also take care of those cases where
  the user does _not_ keep track of mappings and simply uses
  the "unmap from 0 to ~0" each time.

Because of this you could still provide an exact map/unmap logic
and allow such "unmap from 0 to ~0" by making the latter a special
case.
However, if we want to allow any arbitrary/inexact unmap request, then OK.

> > > + if (start <= mlp->daddr &&
> > > +     start + size >= mlp->daddr + NPAGE_TO_SIZE(mlp->npage)) {
> > > +         vfio_dma_unmap(iommu, mlp->daddr, mlp->npage, mlp->rdwr);
> > > +         list_del(&mlp->list);
> > > +         npage_lo = mlp->npage;
> > > +         kfree(mlp);
> > > +         return npage_lo;
> > > + }
> > > +
> > > + /* Overlap low address of existing range */
> >
> > Same as above (ie, '<' is impossible)
> 
> existing:   |<--- A --->|      |<--- B --->|
> unmap:                |<--- C --->|
> 
> Maybe not good practice from userspace, but we shouldn't count on
> userspace to be well behaved.
> 
> > > + if (start <= mlp->daddr) {
> > > +         size_t overlap;
> > > +
> > > +         overlap = start + size - mlp->daddr;
> > > +         npage_lo = overlap >> PAGE_SHIFT;
> > > +         npage_hi = mlp->npage - npage_lo;
> > > +
> > > +         vfio_dma_unmap(iommu, mlp->daddr, npage_lo, mlp->rdwr);
> > > +         mlp->daddr += overlap;
> > > +         mlp->vaddr += overlap;
> > > +         mlp->npage -= npage_lo;
> > > +         return npage_lo;
> > > + }
> >
> > Same as above (ie, '>' is impossible).
> 
> Same example as above.
> 
> > > + /* Overlap high address of existing range */
> > > + if (start + size >= mlp->daddr + NPAGE_TO_SIZE(mlp->npage)) {
> > > +         size_t overlap;
> > > +
> > > +         overlap = mlp->daddr + NPAGE_TO_SIZE(mlp->npage) - start;
> > > +         npage_hi = overlap >> PAGE_SHIFT;
> > > +         npage_lo = mlp->npage - npage_hi;
> > > +
> > > +         vfio_dma_unmap(iommu, start, npage_hi, mlp->rdwr);
> > > +         mlp->npage -= npage_hi;
> > > +         return npage_hi;
> > > + }
> <snip>
> > > +int vfio_dma_map_dm(struct vfio_iommu *iommu, struct vfio_dma_map
> > > *dmp)
> > > +{
> > > + int npage;
> > > + struct dma_map_page *mlp, *mmlp = NULL;
> > > + dma_addr_t daddr = dmp->dmaaddr;
> > > + unsigned long locked, lock_limit, vaddr = dmp->vaddr;
> > > + size_t size = dmp->size;
> > > + int ret = 0, rdwr = dmp->flags & VFIO_DMA_MAP_FLAG_WRITE;
> > > +
> > > + if (vaddr & (PAGE_SIZE-1))
> > > +         return -EINVAL;
> > > + if (daddr & (PAGE_SIZE-1))
> > > +         return -EINVAL;
> > > + if (size & (PAGE_SIZE-1))
> > > +         return -EINVAL;
> > > +
> > > + npage = size >> PAGE_SHIFT;
> > > + if (!npage)
> > > +         return -EINVAL;
> > > +
> > > + if (!iommu)
> > > +         return -EINVAL;
> > > +
> > > + mutex_lock(&iommu->dgate);
> > > +
> > > + if (vfio_find_dma(iommu, daddr, size)) {
> > > +         ret = -EBUSY;
> > > +         goto out_lock;
> > > + }
> > > +
> > > + /* account for locked pages */
> > > + locked = current->mm->locked_vm + npage;
> > > + lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > > + if (locked > lock_limit && !capable(CAP_IPC_LOCK)) {
> > > +         printk(KERN_WARNING "%s: RLIMIT_MEMLOCK (%ld) exceeded\n",
> > > +                 __func__, rlimit(RLIMIT_MEMLOCK));
> > > +         ret = -ENOMEM;
> > > +         goto out_lock;
> > > + }
> > > +
> > > + ret = vfio_dma_map(iommu, daddr, vaddr, npage, rdwr);
> > > + if (ret)
> > > +         goto out_lock;
> > > +
> > > + /* Check if we abut a region below */
> >
> > Is !daddr possible?
> 
> Sure, an IOVA of 0x0.  There's no region below if we start at zero.
> 
> > > + if (daddr) {
> > > +         mlp = vfio_find_dma(iommu, daddr - 1, 1);
> > > +         if (mlp && mlp->rdwr == rdwr &&
> > > +             mlp->vaddr + NPAGE_TO_SIZE(mlp->npage) == vaddr) {
> > > +
> > > +                 mlp->npage += npage;
> > > +                 daddr = mlp->daddr;
> > > +                 vaddr = mlp->vaddr;
> > > +                 npage = mlp->npage;
> > > +                 size = NPAGE_TO_SIZE(npage);
> > > +
> > > +                 mmlp = mlp;
> > > +         }
> > > + }
> >
> > Is !(daddr + size) possible?
> 
> Same, there's no region above if this region goes to the top of the
> address space, ie. 0xffffffff_fffff000 + 0x1000
> 
> Hmm, wonder if I'm missing a check for wrapping.
> 
> > > + if (daddr + size) {
> > > +         mlp = vfio_find_dma(iommu, daddr + size, 1);
> > > +         if (mlp && mlp->rdwr == rdwr && mlp->vaddr == vaddr + size)
> > > {
> > > +
> > > +                 mlp->npage += npage;
> > > +                 mlp->daddr = daddr;
> > > +                 mlp->vaddr = vaddr;
> > > +
> > > +                 /* If merged above and below, remove previously
> > > +                  * merged entry.  New entry covers it.  */
> > > +                 if (mmlp) {
> > > +                         list_del(&mmlp->list);
> > > +                         kfree(mmlp);
> > > +                 }
> > > +                 mmlp = mlp;
> > > +         }
> > > + }
> > > +
> > > + if (!mmlp) {
> > > +         mlp = kzalloc(sizeof *mlp, GFP_KERNEL);
> > > +         if (!mlp) {
> > > +                 ret = -ENOMEM;
> > > +                 vfio_dma_unmap(iommu, daddr, npage, rdwr);
> > > +                 goto out_lock;
> > > +         }
> > > +
> > > +         mlp->npage = npage;
> > > +         mlp->daddr = daddr;
> > > +         mlp->vaddr = vaddr;
> > > +         mlp->rdwr = rdwr;
> > > +         list_add(&mlp->list, &iommu->dm_list);
> > > + }
> > > +
> > > +out_lock:
> > > + mutex_unlock(&iommu->dgate);
> > > + return ret;
> > > +}
> > > +
> > > +static int vfio_iommu_release(struct inode *inode, struct file
> *filep)
> > > +{
> > > + struct vfio_iommu *iommu = filep->private_data;
> > > +
> > > + vfio_release_iommu(iommu);
> > > + return 0;
> > > +}
> > > +
> > > +static long vfio_iommu_unl_ioctl(struct file *filep,
> > > +                          unsigned int cmd, unsigned long arg)
> > > +{
> > > + struct vfio_iommu *iommu = filep->private_data;
> > > + int ret = -ENOSYS;
> >
> > Any reason for not using "switch" ?
> 
> It got ugly in vfio_main, so I decided to be consistent w/ it in the
> driver and use if/else here too.  I don't like the aesthetics of extra
> {}s to declare variables within a switch, nor do I like declaring all
> the variables for each case for the whole function.  Personal quirk.
> 
> > > +        if (cmd == VFIO_IOMMU_GET_FLAGS) {
> > > +                u64 flags = VFIO_IOMMU_FLAGS_MAP_ANY;
> > > +
> > > +                ret = put_user(flags, (u64 __user *)arg);
> > > +
> > > +        } else if (cmd == VFIO_IOMMU_MAP_DMA) {
> > > +         struct vfio_dma_map dm;
> > > +
> > > +         if (copy_from_user(&dm, (void __user *)arg, sizeof dm))
> > > +                 return -EFAULT;
> >
> > What does the "_dm" suffix stand for?
> 
> Inherited from Tom, but I figure _dma_map_dm = action(dma map),
> object(dm), which is a vfio_Dma_Map.

OK. The reason why I asked is that '_dm' does not add anything to 
'vfio_dma_map'.

/Chris