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

Sean Christopherson
2024-02-19T22:04:06Z

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.