Re: [Qemu-devel] [PATCH] RFC: vmcoreinfo device

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 02 May 2017 19:03:15 +0000
Marc-André Lureau <marcandre.lureau@gmail.com> wrote:

> Hi
> 
> On Tue, May 2, 2017 at 11:17 AM Igor Mammedov <imammedo@redhat.com> wrote:
> 
> > On Fri, 28 Apr 2017 14:28:38 +0000
> > Marc-André Lureau <marcandre.lureau@gmail.com> wrote:
> >  
> > > Hi
> > >
> > > On Fri, Apr 28, 2017 at 6:12 PM Ladi Prosek <lprosek@redhat.com> wrote:
> > >  
> > > > On Mon, Apr 24, 2017 at 3:03 PM, Marc-André Lureau
> > > > <marcandre.lureau@redhat.com> wrote:  
> > > > > The VM coreinfo (vmcoreinfo) device is an emulated device which
> > > > > exposes a 4k memory range to the guest to store various informations
> > > > > useful to debug the guest OS. (it is greatly inspired by the VMGENID
> > > > > device implementation)
> > > > >
> > > > > This is an early-boot alternative to the qemu-ga VMDUMP_INFO event
> > > > > proposed in "[PATCH 00/21] WIP: dump: add kaslr support".
> > > > >
> > > > > If deemed more appropriate, we can consider writing to fw_cfg  
> > directly  
> > > > > instead of guest memory, now that qemu 2.9 supports it again.
> > > > >
> > > > > The proof-of-concept kernel module:
> > > > > https://github.com/elmarco/vmgenid-test/blob/master/qemuvmci-test.c  
> > > >
> > > > Here's a proof-of-concept Windows driver:
> > > >
> > > >  
> > https://github.com/ladipro/kvm-guest-drivers-windows/tree/vmcoreinfo/vmcoreinfo  
> > > >
> > > > I just wanted to be sure that it's possible to evaluate the ADDR
> > > > method in Windows.
> > > >
> > > > From a practical point of view it is unfortunate that this would be a
> > > > completely new device. For Windows guests it means another driver
> > > > binary and all the overhead associated with deploying it on VMs. Would
> > > > it be too crazy to add this functionality to the pvpanic device? The
> > > > mechanics could stay the same but it would be done under the existing
> > > > ACPI\QEMU0001 device.
> > > >  
> > >
> > > pvpanic is under _SB.PCI0.ISA, that could be problematic
> > >
> > > and _STA is a name field.
> > >
> > > Someone with more experience with ACPI could tell us if that make sense  
> > to  
> > > merge both and how.
> > >
> > > Can't you handle 2 ACPI devices in the same windows driver instead?  
> > we use QEMU0001 to reserve IO ports for pvpanic device,
> > ASL wise there shouldn't problems with adding _ADDR method to it
> >
> > but then we probably should fold vmcoreinfo into pvpanic device
> > as well (QEMU and linux driver)
> >
> >  
> pvpanic is x86-only afaict.
There is nothing that forces it to be x86 specific (beside being ISA device),
ARM also can benefit from/use pvpanic if you make it pci device or just plain
Device.

> While I think vmcoreinfo would work fine with
> any acpi-able arch.
I don't insist on it but it's worth a try, probably a lot of code could be
shared between both (including AML part/which makes DSDT smaller little bit)


> I think I would rather modify the windows driver to support both pvpanic &
> vmcoreinfo, even if it's not typical for driver to implement several
> devices.
> 
> >  
> > > > +Storage Format:  
> > > > > +---------------
> > > > > +
> > > > > +The content is expected to use little-endian format.
> > > > > +
> > > > > +In order to implement an OVMF "SDT Header Probe Suppressor", the  
> > > > contents of  
> > > > > +the vmcoreinfo blob has 40 bytes of padding:
> > > > > +
> > > > > ++-----------------------------------+
> > > > > +| SSDT with OEM Table ID = VMCOREIN |
> > > > > ++-----------------------------------+
> > > > > +| ...                               |       TOP OF PAGE
> > > > > +| VCIA dword object ----------------|----->  
> > > > +---------------------------+  
> > > > > +| ...                               |       | fw-allocated array for  
> > > > |  
> > > > > +| _STA method referring to VCIA     |       | "etc/vmcoreinfo"  
> > > > |  
> > > > > +| ...                               |  
> > > >  +---------------------------+  
> > > > > +| ADDR method referring to VCIA     |       |  0: OVMF SDT Header  
> > probe  
> > > > |  
> > > > > +| ...                               |       |     suppressor  
> > > > |  
> > > > > ++-----------------------------------+       | 40: uint32 version  
> > field  
> > > > |  
> > > > > +                                            | 44: info contents  
> > > >  |  
> > > > > +                                            |     ....  
> > > > |  
> > > > > +  
> > > > +---------------------------+  
> > > > > +                                            END OF PAGE
> > > > > +
> > > > > +Version 0 content:
> > > > > +
> > > > > + uint64 paddr:
> > > > > +  Physical address of the Linux vmcoreinfo ELF note.  
> > > >
> > > > Or physical address of the Windows crash dump header :p
> > > >  
> > >
> > > Is there support for Windows crash dump in qemu?
> > >
> > >  
> > > > Do we want to have an additional discriminator field to tell what kind
> > > > of information was written by the guest or would Windows use a
> > > > different version?
> > > >
> > > >  
> > > I guess a different version would be ok.
> > >
> > > Thanks a lot for looking at it!  
> >
> > --  
> Marc-André Lureau