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

From: Sven Schnelle <svens@linux.ibm.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	linux-kernel@vger.kernel.org, aleksandar.qemu.devel@gmail.com,
	alexandru.elisei@arm.com, anup.patel@wdc.com,
	aou@eecs.berkeley.edu, atish.patra@wdc.com,
	benh@kernel.crashing.org, bp@alien8.de, catalin.marinas@arm.com,
	chenhuacai@kernel.org, dave.hansen@linux.intel.com,
	david@redhat.com, frankja@linux.ibm.com, frederic@kernel.org,
	gor@linux.ibm.com, hca@linux.ibm.com, imbrenda@linux.ibm.com,
	james.morse@arm.com, jmattson@google.com, joro@8bytes.org,
	kvm@vger.kernel.org, maz@kernel.org, mingo@redhat.com,
	mpe@ellerman.id.au, nsaenzju@redhat.com, palmer@dabbelt.com,
	paulmck@kernel.org, paulus@samba.org, paul.walmsley@sifive.com,
	seanjc@google.com, suzuki.poulose@arm.com, tglx@linutronix.de,
	tsbogend@alpha.franken.de, vkuznets@redhat.com,
	wanpengli@tencent.com, will@kernel.org
Subject: Re: [PATCH 0/5] kvm: fix latent guest entry/exit bugs
Date: Wed, 19 Jan 2022 07:41:56 +0100	[thread overview]
Message-ID: <yt9dbl08uszv.fsf@linux.ibm.com> (raw)
In-Reply-To: <20220118175051.GE17938@C02TD0UTHF1T.local> (Mark Rutland's message of "Tue, 18 Jan 2022 17:50:51 +0000")

Hi Mark,

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

> On Tue, Jan 18, 2022 at 05:09:25PM +0100, Sven Schnelle wrote:
>> Mark Rutland <mark.rutland@arm.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