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Re: [Qemu-devel] [PATCH] kvm: pass the virtual SEI syndrome to guest OS

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH] kvm: pass the virtual SEI syndrome to guest OS
Date: Wed, 29 Mar 2017 16:54:41 +0300

On Wed, Mar 29, 2017 at 03:36:59PM +0200, Laszlo Ersek wrote:
> On 03/29/17 14:51, Michael S. Tsirkin wrote:
> > On Wed, Mar 29, 2017 at 01:58:29PM +0200, Laszlo Ersek wrote:
> >> (8) When QEMU gets SIGBUS from the kernel -- I hope that's going to come
> >> through a signalfd -- QEMU can format the CPER right into guest memory,
> >> and then inject whatever interrupt (or assert whatever GPIO line) is
> >> necessary for notifying the guest.
> > 
> > I think I see a race condition potential - what if guest accesses
> > CPER in guest memory while it's being written?
> I'm not entirely sure about the data flow here (these parts of the ACPI
> spec are particularly hard to read...), but I thought the OS wouldn't
> look until it got a notification.

There could be multiple notifications, OS might be looking
there because of them.

> Or, are you concerned about the next CPER write by QEMU, while the OS is
> reading the last one (and maybe the CPER area could wrap around?)
> > 
> > We can probably use another level of indirection to fix this:
> > 
> > allocate twice the space, add a pointer to where the valid
> > table is located and update that after writing CPER completely.
> > The pointer can be written atomically but also needs to
> > be read atomically, so I suspect it should be a single byte
> > as we don't know how are OSPMs implementing this.
> > 
> A-B-A problem? (Is that usually solved with a cookie or a wider
> generation counter? But that would again require wider atomics.)
> I do wonder though how this is handled on physical hardware. Assuming
> the hardware error traps to the firmware first (which, on phys hw, is
> responsible for depositing the CPER), in that scenario the phys firmware
> would face the same issue (i.e., asynchronously interrupting the OS,
> which could be reading the previously stored CPER).
> Thanks,
> Laszlo

ACPI spec seems to specify a set of serialization actions. I'm guessing
this is what you need to use to avoid changing guest state
while it's reading entries.


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