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

From: Christian Borntraeger <borntraeger@linux.ibm.com>
To: Mark Rutland <mark.rutland@arm.com>,
	Sven Schnelle <svens@linux.ibm.com>,
	Heiko Carstens <hca@linux.ibm.com>,
	linux-s390 <linux-s390@vger.kernel.org>
Cc: 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: Tue, 18 Jan 2022 15:15:51 +0100	[thread overview]
Message-ID: <77e8d214-372b-3f0e-7b4e-5c2d23a4199c@linux.ibm.com> (raw)
In-Reply-To: <20220118131223.GC17938@C02TD0UTHF1T.local>

Am 18.01.22 um 14:12 schrieb Mark Rutland:
> 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.
What exactly would the low-level interrupt handler need to do?

CC Sven, Heiko for the entry.S changes.