Re: [Qemu-devel] Virtual Machine Generation ID

[Igor Mammedov]

On Mon, 16 Jan 2017 10:57:42 -0800
Ben Warren <address@hidden> wrote:

> > On Jan 16, 2017, at 6:21 AM, Michael S. Tsirkin <address@hidden> wrote:
> > 
> > On Sat, Jan 14, 2017 at 10:17:53PM -0800, Ben Warren wrote:  
> >> Hi Michael,
> >> 
> >> 
> >>    On Dec 10, 2016, at 7:28 PM, Michael S. Tsirkin <address@hidden> wrote:
> >> 
> >>    On Tue, Dec 06, 2016 at 06:15:34PM -0800, Ben Warren wrote:
> >> 
> >>        Hi Michael,
> >> 
> >>        I’m well on my way to implementing this, but I am really new to the
> >>        QEMU code base and am struggling with some concepts.  Please see 
> >> below:
> >> 
> >>            On Oct 5, 2016, at 6:29 PM, Michael S. Tsirkin <address@hidden>
> >>            wrote:
> >> 
> >>            On Tue, Oct 04, 2016 at 03:51:40PM -0700, Ed Swierk wrote:
> >> 
> >>                On Thu, Sep 15, 2016 at 5:36 PM, Michael S. Tsirkin <  
> >>                address@hidden> wrote:  
> >> 
> >>                    On Thu, Sep 15, 2016 at 05:23:28PM -0700, Ed Swierk 
> >> wrote:
> >> 
> >>                        I'm wondering what it will take to finish up work on
> >>                        vmgenid.
> >> 
> >>                        
> >> https://lists.gnu.org/archive/html/qemu-devel/2016-01/
> >>                        msg05599.html
> >> 
> >> 
> >>                    We have ACPI_BUILD_TPMLOG_FILE in tree now and I think 
> >> it
> >>                    could be
> >>                    allocated in a similar way.
> >>                    Integrate patch "fw-cfg: support writeable blobs" to
> >>                    communicate the
> >>                    allocated address back to QEMU.
> >> 
> >> 
> >>                Starting with Igor's last version at
> >>                https://github.com/imammedo/qemu/commits/vmgen_wip , it's 
> >> not
> >>                clear to
> >>                me which changes need to be ported, which changes are 
> >> obsoleted
> >>                by
> >>                your new fw-cfg stuff and/or upstream churn in ACPI, device
> >>                properties, etc. In particular ACPI is still a total 
> >> mystery to
> >>                me,
> >>                though passing a single address from guest to host can't be
> >>                that hard,
> >>                can it?
> >> 
> >>                Any clues would be appreciated.
> >> 
> >>                --Ed
> >> 
> >> 
> >>            It might be best to just re-start from the beginning.
> >>            So the idea is that ACPI should be about supplying the address
> >>            to guest. To supply address to host we'll use fw cfg.
> >>            This would be new I think:
> >> 
> >>            - add support for writeable fw cfg blobs
> >> 
> >>        patch applied
> >> 
> >>            - add linker/loader command to write address of a blob into
> >>            such a fw cfg file
> >>            - add a new file used for vm gen id, use loader command above
> >>            to pass the address of a blob allocated for it to host
> >> 
> >>        I don’t really understand the meaning of “file” in this context.  It
> >>        seems to be a way of specifying individual fw_cfg entries without
> >>        explicitly giving an index, but is not something that is visible in
> >>        either the host or guest file system.  Is this about right?  In my 
> >> code
> >>        I’m using “/etc/vmgenid”
> >> 
> >> 
> >>    yes
> >> 
> >> 
> >>        As for the blob, I’m thinking this is where my main problem is.  The
> >>        ‘fw_cfg_add_*()’ functions take a data pointer but doesn’t seem to 
> >> copy
> >>        the data anywhere.  We pass essentially a pointer via ACPI to the
> >>        guest, so what it points to needs to be in an accessible region.  I
> >>        don’t get how to define the blob contents.  There are command-line
> >>        ‘fw-cfg’ options where you can specify a file, but it’s not clear 
> >> to me
> >>        how to use them.  Maybe I reserve some IO memory or something?
> >> 
> >> 
> >>    Not sure I understand the question. fw cfg device will make
> >>    memory accessible to guest. Put the guest physical address there.
> >>    the address needs to be calculated by linker.
> >> 
> >> 
> >> I’m almost ready to submit a V2 of the patch set, but there’s still one 
> >> issue
> >> that I can’t figure out.  From the guest, I can read the contents of the 
> >> blob.
> >> If I make a change to the contents of the blob (via QMP) the guest does not
> >> see the changes.  Is there something I need to do on the QEMU side to 
> >> “push”
> >> the updated fw_cfg contents to the guest?  I’ve noticed this both when 
> >> writing
> >> a qtest for the feature, and also in a Linux kernel module I wrote that 
> >> reads
> >> the ACPI contents in a guest.
> >> 
> >> thanks,
> >> Ben  
> > 
> > fw cfg entities are assumed to be immutable. This week
> > I'll merge support for writeable fw cfg entries.
> > I don't see why you want to change fw cfg transparently
> > though - I think it should be like this
> > - guest writes GPA into fw cfg
> > - qemu writes gen id at this GPA
> >   
> I’ve tried with your patch "fw-cfg-support-writeable-blobs”, setting the 
> ‘read-only’ flag on my blob to false:
> 
> fw_cfg_add_file_callback(s, VMGENID_FW_CFG_FILE, NULL, NULL, vms->guid.data, 
> sizeof(vms->guid.data), false);
> 
> and it doesn’t seem to make a difference.  
> 
> I think we have a misunderstanding here.  I’m storing the VM Generation ID 
> __data__ (a GUID) in a fw_cfg blob, not the address.  I’ll submit the patch 
> set ASAP so you will understand.
there  should be another fw_cfg file for address so
that guest would be able to write it back into QEMU

> 
> > -- 
> > MST  
> 
> regards,
> Ben
>