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    Subject[PATCH v13 20/21] KVM: x86/xen: allow vcpu_info content to be 'safely' copied
    Date
    From: Paul Durrant <pdurrant@amazon.com>

    If the guest sets an explicit vcpu_info GPA then, for any of the first 32
    vCPUs, the content of the default vcpu_info in the shared_info page must be
    copied into the new location. Because this copy may race with event
    delivery (which updates the 'evtchn_pending_sel' field in vcpu_info) there
    needs to be a way to defer that until the copy is complete.
    Happily there is already a shadow of 'evtchn_pending_sel' in kvm_vcpu_xen
    that is used in atomic context if the vcpu_info PFN cache has been
    invalidated so that the update of vcpu_info can be deferred until the
    cache can be refreshed (on vCPU thread's the way back into guest context).

    Also use this shadow if the vcpu_info cache has been *deactivated*, so that
    the VMM can safely copy the vcpu_info content and then re-activate the
    cache with the new GPA. To do this, stop considering an inactive vcpu_info
    cache as a hard error in kvm_xen_set_evtchn_fast().

    Signed-off-by: Paul Durrant <pdurrant@amazon.com>
    Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
    ---
    Cc: David Woodhouse <dwmw2@infradead.org>
    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: x86@kernel.org

    v13:
    - Patch title change.

    v8:
    - Update commit comment.

    v6:
    - New in this version.
    ---
    arch/x86/kvm/xen.c | 3 ---
    1 file changed, 3 deletions(-)

    diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
    index 8650141b266e..11ab62ca011d 100644
    --- a/arch/x86/kvm/xen.c
    +++ b/arch/x86/kvm/xen.c
    @@ -1802,9 +1802,6 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
    WRITE_ONCE(xe->vcpu_idx, vcpu->vcpu_idx);
    }

    - if (!vcpu->arch.xen.vcpu_info_cache.active)
    - return -EINVAL;
    -
    if (xe->port >= max_evtchn_port(kvm))
    return -EINVAL;

    --
    2.39.2

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