On Tue, Jan 17, 2017 at 05:24:04PM +0100, Igor Mammedov wrote: > On Tue, 17 Jan 2017 07:01:27 -0800 > Ed Swierk <eswi...@skyportsystems.com> wrote: > > > 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? > pls note that acpi_ram_update() updates only memory on qemu side and > it's not mapped into guest. Updated memory will be re-read by firmware > when guest reboots. > > But with vmgenid, QEMU might want to update guest RAM which > contains GUID without reboot, and for this it needs to know GPA > of GUID buffer and only after updating it send ACPI event.
+1 thanks, thanks is what I was trying to say. > BTW in-place update is racy anyways as OS could be reading it > at the same time, That would be up to the applications to use correctly. E.g. check id send transaction ---> racy send transaction check id ---> not racy I can't of course answer for it that all applications use the id correctly. Some of them might decide to accept the risk of a race condition. > but we can't do anything about it as vmgenid > spec didn't provide means for atomic update. I agree it's not up to us to fix. > > > --Ed > > > > > > On Tue, Jan 17, 2017 at 6:33 AM, Michael S. Tsirkin <m...@redhat.com> 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 <imamm...@redhat.com> > > >> wrote: > > >> > On Mon, 16 Jan 2017 10:57:42 -0800 > > >> > Ben Warren <b...@skyportsystems.com> wrote: > > >> > > > >> >> > On Jan 16, 2017, at 6:21 AM, Michael S. Tsirkin <m...@redhat.com> > > >> >> > 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 > > >> >> >> <m...@redhat.com> 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 > > >> >> >> <m...@redhat.com> > > >> >> >> 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 > > >> >> >> < > > >> >> >> m...@redhat.com> 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 > > >> >> > > >> >