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    Subject[PATCH v11 17/19] KVM: xen: don't block on pfncache locks in kvm_xen_set_evtchn_fast()
    Date
    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

    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 4ebd24c3ae70..845ff23c8399 100644
    --- a/arch/x86/kvm/xen.c
    +++ b/arch/x86/kvm/xen.c
    @@ -1675,10 +1675,13 @@ 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;

    + if (!kvm_gpc_check(gpc, PAGE_SIZE))
    + goto out_unlock;
    +
    if (IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode) {
    struct shared_info *shinfo = gpc->khva;

    @@ -1700,8 +1703,10 @@ static int set_shinfo_evtchn_pending(struct kvm_vcpu *vcpu, u32 port)
    rc = 1; /* It is newly raised */
    }

    + out_unlock:
    + read_unlock(&gpc->lock);
    out:
    - read_unlock_irqrestore(&gpc->lock, flags);
    + local_irq_restore(flags);
    return rc;
    }

    @@ -1711,21 +1716,23 @@ static bool set_vcpu_info_evtchn_pending(struct kvm_vcpu *vcpu, u32 port)
    struct gfn_to_pfn_cache *gpc = &vcpu->arch.xen.vcpu_info_cache;
    unsigned long flags;
    bool kick_vcpu = false;
    + bool locked;

    - read_lock_irqsave(&gpc->lock, flags);
    + local_irq_save(flags);
    + locked = read_trylock(&gpc->lock);

    /*
    * Try to deliver the event directly to the vcpu_info. If successful and
    * the guest is using upcall_vector delivery, send the MSI.
    - * If the pfncache is invalid, set the shadow. In this case, or if the
    - * guest is using another form of event delivery, the vCPU must be
    - * kicked to complete the delivery.
    + * If the pfncache lock is contended or the cache is invalid, set the
    + * shadow. In this case, or if the guest is using another form of event
    + * delivery, the vCPU must be kicked to complete the delivery.
    */
    if (IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode) {
    struct vcpu_info *vcpu_info = gpc->khva;
    int port_word_bit = port / 64;

    - if (!kvm_gpc_check(gpc, sizeof(*vcpu_info))) {
    + if ((!locked || !kvm_gpc_check(gpc, sizeof(*vcpu_info)))) {
    if (!test_and_set_bit(port_word_bit, &vcpu->arch.xen.evtchn_pending_sel))
    kick_vcpu = true;
    goto out;
    @@ -1739,7 +1746,7 @@ static bool set_vcpu_info_evtchn_pending(struct kvm_vcpu *vcpu, u32 port)
    struct compat_vcpu_info *vcpu_info = gpc->khva;
    int port_word_bit = port / 32;

    - if (!kvm_gpc_check(gpc, sizeof(*vcpu_info))) {
    + if ((!locked || !kvm_gpc_check(gpc, sizeof(*vcpu_info)))) {
    if (!test_and_set_bit(port_word_bit, &vcpu->arch.xen.evtchn_pending_sel))
    kick_vcpu = true;
    goto out;
    @@ -1758,7 +1765,10 @@ static bool set_vcpu_info_evtchn_pending(struct kvm_vcpu *vcpu, u32 port)
    }

    out:
    - read_unlock_irqrestore(&gpc->lock, flags);
    + if (locked)
    + read_unlock(&gpc->lock);
    +
    + local_irq_restore(flags);
    return kick_vcpu;
    }

    --
    2.39.2

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