linux-kernel - Re: [PATCH 0/5] kvm: fix latent guest entry/exit bugs

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Date:   Tue, 18 Jan 2022 17:09:25 +0100
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org, aleksandar.qemu.devel@...il.com,
        alexandru.elisei@....com, anup.patel@....com,
        aou@...s.berkeley.edu, atish.patra@....com,
        benh@...nel.crashing.org, bp@...en8.de, catalin.marinas@....com,
        chenhuacai@...nel.org, dave.hansen@...ux.intel.com,
        david@...hat.com, frankja@...ux.ibm.com, frederic@...nel.org,
        gor@...ux.ibm.com, hca@...ux.ibm.com, imbrenda@...ux.ibm.com,
        james.morse@....com, jmattson@...gle.com, joro@...tes.org,
        kvm@...r.kernel.org, maz@...nel.org, mingo@...hat.com,
        mpe@...erman.id.au, nsaenzju@...hat.com, palmer@...belt.com,
        paulmck@...nel.org, paulus@...ba.org, paul.walmsley@...ive.com,
        seanjc@...gle.com, suzuki.poulose@....com, tglx@...utronix.de,
        tsbogend@...ha.franken.de, vkuznets@...hat.com,
        wanpengli@...cent.com, will@...nel.org
Subject: Re: [PATCH 0/5] kvm: fix latent guest entry/exit bugs

Hi Mark,

Mark Rutland <mark.rutland@....com> writes:

> On Tue, Jan 18, 2022 at 01:42:26PM +0100, Christian Borntraeger wrote:
>> 
>> 
>> Am 18.01.22 um 13:02 schrieb Mark Rutland:
>> > On Mon, Jan 17, 2022 at 06:45:36PM +0100, Paolo Bonzini wrote:
>> > > On 1/14/22 16:19, Mark Rutland wrote:
>> > > > I also think there is another issue here. When an IRQ is taken from SIE, will
>> > > > user_mode(regs) always be false, or could it be true if the guest userspace is
>> > > > running? If it can be true I think tha context tracking checks can complain,
>> > > > and it*might*  be possible to trigger a panic().
>> > > 
>> > > I think that it would be false, because the guest PSW is in the SIE block
>> > > and switched on SIE entry and exit, but I might be incorrect.
>> > 
>> > Ah; that's the crux of my confusion: I had thought the guest PSW would
>> > be placed in the regular lowcore *_old_psw slots. From looking at the
>> > entry asm it looks like the host PSW (around the invocation of SIE) is
>> > stored there, since that's what the OUTSIDE + SIEEXIT handling is
>> > checking for.
>> > 
>> > Assuming that's correct, I agree this problem doesn't exist, and there's
>> > only the common RCU/tracing/lockdep management to fix.
>> 
>> Will you provide an s390 patch in your next iteration or shall we then do
>> one as soon as there is a v2? We also need to look into vsie.c where we
>> also call sie64a
>
> I'm having a go at that now; my plan is to try to have an s390 patch as
> part of v2 in the next day or so.
>
> Now that I have a rough idea of how SIE and exception handling works on
> s390, I think the structural changes to kvm-s390.c:__vcpu_run() and
> vsie.c:do_vsie_run() are fairly simple.
>
> The only open bit is exactly how/where to identify when the interrupt
> entry code needs to wake RCU. I can add a per-cpu variable or thread
> flag to indicate that we're inside that EQS, or or I could move the irq
> enable/disable into the sie64a asm and identify that as with the OUTSIDE
> macro in the entry asm.

I wonder whether the code in irqentry_enter() should call a function
is_eqs() instead of is_idle_task(). The default implementation would
be just a

#ifndef is_eqs
#define is_eqs is_idle_task
#endif

and if an architecture has special requirements, it could just define
is_eqs() and do the required checks there. This way the architecture
could define whether it's a percpu bit, a cpu flag or something else.

/Sven