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Date: Mon, 19 Feb 2024 14:04:04 -0800
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Subject: Re: [PATCH v13 18/21] KVM: x86/xen: don't block on pfncache locks in kvm_xen_set_evtchn_fast()
From: Sean Christopherson <seanjc@google.com>
To: Paul Durrant <paul@xen.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Jonathan Corbet <corbet@lwn.net>, 
	Christian Borntraeger <borntraeger@linux.ibm.com>, Janosch Frank <frankja@linux.ibm.com>, 
	Claudio Imbrenda <imbrenda@linux.ibm.com>, David Hildenbrand <david@redhat.com>, 
	Heiko Carstens <hca@linux.ibm.com>, Vasily Gorbik <gor@linux.ibm.com>, 
	Alexander Gordeev <agordeev@linux.ibm.com>, Sven Schnelle <svens@linux.ibm.com>, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org, 
	"H. Peter Anvin" <hpa@zytor.com>, David Woodhouse <dwmw2@infradead.org>, Shuah Khan <shuah@kernel.org>, 
	kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, 
	linux-s390@vger.kernel.org, linux-kselftest@vger.kernel.org
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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.