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 >> >
