Re: [PATCH v13 18/21] KVM: x86/xen: don't block on pfncache locks in kvm_xen_set_evtchn_fast()

Paul Durrant
2024-02-20T09:05:33Z

On 19/02/2024 22:04, Sean Christopherson wrote:
> On Thu, Feb 15, 2024, Paul Durrant wrote:
>> From: Paul Durrant <pdurrant@amazon.com>
>>
>> As described in [1] compiling with CONFIG_PROVE_RAW_LOCK_NESTING shows that
>> kvm_xen_set_evtchn_fast() is blocking on pfncache locks in IRQ context.
>> There is only actually blocking with PREEMPT_RT because the locks will
>> turned into mutexes. There is no 'raw' version of rwlock_t that can be used
>> to avoid that, so use read_trylock() and treat failure to lock the same as
>> an invalid cache.
>>
>> [1] https://lore.kernel.org/lkml/99771ef3a4966a01fefd3adbb2ba9c3a75f97cf2.camel@infradead.org/T/#mbd06e5a04534ce9c0ee94bd8f1e8d942b2d45bd6
>>
>> Signed-off-by: Paul Durrant <pdurrant@amazon.com>
>> Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
>> ---
>> Cc: Sean Christopherson <seanjc@google.com>
>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Borislav Petkov <bp@alien8.de>
>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: David Woodhouse <dwmw2@infradead.org>
>> Cc: x86@kernel.org
>>
>> v13:
>> - Patch title change.
>>
>> v11:
>> - Amended the commit comment.
>>
>> v10:
>> - New in this version.
>> ---
>> arch/x86/kvm/xen.c | 30 ++++++++++++++++++++----------
>> 1 file changed, 20 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
>> index 59073642c078..8650141b266e 100644
>> --- a/arch/x86/kvm/xen.c
>> +++ b/arch/x86/kvm/xen.c
>> @@ -1678,10 +1678,13 @@ static int set_shinfo_evtchn_pending(struct kvm_vcpu *vcpu, u32 port)
>> unsigned long flags;
>> int rc = -EWOULDBLOCK;
>>
>> - read_lock_irqsave(&gpc->lock, flags);
>> - if (!kvm_gpc_check(gpc, PAGE_SIZE))
>> + local_irq_save(flags);
>> + if (!read_trylock(&gpc->lock))
>> goto out;
>
> I am not comfortable applying this patch. As shown by the need for the next patch
> to optimize unrelated invalidations, switching to read_trylock() is more subtle
> than it seems at first glance. Specifically, there are no fairness guarantees.
>
> I am not dead set against this change, but I don't want to put my SoB on what I
> consider to be a hack.
>
> I've zero objections if you can convince Paolo to take this directly, i.e. this
> isn't a NAK. I just don't want to take it through my tree.

Ok. I'll drop this from v14 then. It can go separately, assuming there
is no move to add the raw lock which would negate it.