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

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Message-ID: <20220118175051.GE17938@C02TD0UTHF1T.local>
Date:   Tue, 18 Jan 2022 17:50:51 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Sven Schnelle <svens@...ux.ibm.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,
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        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

On Tue, Jan 18, 2022 at 05:09:25PM +0100, Sven Schnelle wrote:
> Hi Mark,

Hi Sven,

> 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.

In the case of checking is_idle_thread() and checking for PF_VCPU, I'm
assuming the compiler can merge the loads of current->flags, and there's
little gain by making this entirely architecture specific, but we can
always check that and/or reconsider in future.

Thanks,
Mark.