Re: [Qemu-devel] Virtual Machine Generation ID

[Ed Swierk]

You mean what causes the guest to re-read the vmgenid guid? The
vmgenid ACPI table defines a notify method, and when the guest
receives the corresponding event, it re-reads the guid. (Also it
appears that with Windows Server 2012 at least, if no notify method is
defined, as is the case with Xen, the guest just re-reads it on
demand.)

Wouldn't it be sufficient for the qmp set-vmgenid command to call
acpi_ram_update() with the new guid, and acpi_send_event() to notify
the guest?

--Ed


On Tue, Jan 17, 2017 at 6:33 AM, Michael S. Tsirkin <address@hidden> wrote:
> Because this relies on guest to run code to read out the new fw cfg.
> Thus guest can not reliably detect that host didn't update the gen id -
> new one might be there in fw cfg but not yet read.
>
> RSDP never changes during guest lifetime so the issue does
> not occur.
>
> On Tue, Jan 17, 2017 at 06:26:57AM -0800, Ed Swierk wrote:
>> Why is using fw_cfg for vmgenid preferable to the approach used for
>> RSDP: call acpi_add_rom_blob() to allocate a MemoryRegion with the
>> initial vmgenid guid, and call acpi_ram_update() with the new guid
>> whenever the host needs to change it?
>>
>> --Ed
>>
>>
>> On Tue, Jan 17, 2017 at 5:26 AM, Igor Mammedov <address@hidden> wrote:
>> > 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
>> >>
>> >