Re: [Qemu-devel] Virtual Machine Generation ID

[Ed Swierk]

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
>>
>