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

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Date:   Wed, 19 Jan 2022 07:41:56 +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 05:09:25PM +0100, Sven Schnelle wrote:
>> Mark Rutland <mark.rutland@....com> writes:
>> > On Tue, Jan 18, 2022 at 01:42:26PM +0100, Christian Borntraeger wrote:
>> >> 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.
>
> I had come to almost the same approach: I've added an arch_in_rcu_eqs()
> which is checked in addition to the existing is_idle_thread() check.
>

Sounds good, thanks!

/Sven