linux-kernel - Re: [PATCH] kvm: pass the virtual SEI syndrome to guest OS

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Message-ID: <20170329164811-mutt-send-email-mst@kernel.org>
Date:   Wed, 29 Mar 2017 16:54:41 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Laszlo Ersek <lersek@...hat.com>
Cc:     Achin Gupta <achin.gupta@....com>,
        gengdongjiu <gengdongjiu@...wei.com>, ard.biesheuvel@...aro.org,
        edk2-devel@...ts.01.org, qemu-devel@...gnu.org,
        zhaoshenglong@...wei.com, James Morse <james.morse@....com>,
        Christoffer Dall <cdall@...aro.org>, xiexiuqi@...wei.com,
        Marc Zyngier <marc.zyngier@....com>, catalin.marinas@....com,
        will.deacon@....com, christoffer.dall@...aro.org,
        rkrcmar@...hat.com, suzuki.poulose@....com, andre.przywara@....com,
        mark.rutland@....com, vladimir.murzin@....com,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        wangxiongfeng2@...wei.com, wuquanming@...wei.com,
        huangshaoyu@...wei.com, Leif.Lindholm@...aro.com, nd@....com,
        Igor Mammedov <imammedo@...hat.com>
Subject: Re: [PATCH] kvm: pass the virtual SEI syndrome to guest OS

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.

-- 
MST