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    Subject[PATCH 5.10 043/132] KVM: x86: Wait for IPIs to be delivered when handling Hyper-V TLB flush hypercall
    Date
    From: Vitaly Kuznetsov <vkuznets@redhat.com>

    commit 1ebfaa11ebb5b603a3c3f54b2e84fcf1030f5a14 upstream.

    Prior to commit 0baedd792713 ("KVM: x86: make Hyper-V PV TLB flush use
    tlb_flush_guest()"), kvm_hv_flush_tlb() was using 'KVM_REQ_TLB_FLUSH |
    KVM_REQUEST_NO_WAKEUP' when making a request to flush TLBs on other vCPUs
    and KVM_REQ_TLB_FLUSH is/was defined as:

    (0 | KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)

    so KVM_REQUEST_WAIT was lost. Hyper-V TLFS, however, requires that
    "This call guarantees that by the time control returns back to the
    caller, the observable effects of all flushes on the specified virtual
    processors have occurred." and without KVM_REQUEST_WAIT there's a small
    chance that the vCPU making the TLB flush will resume running before
    all IPIs get delivered to other vCPUs and a stale mapping can get read
    there.

    Fix the issue by adding KVM_REQUEST_WAIT flag to KVM_REQ_TLB_FLUSH_GUEST:
    kvm_hv_flush_tlb() is the sole caller which uses it for
    kvm_make_all_cpus_request()/kvm_make_vcpus_request_mask() where
    KVM_REQUEST_WAIT makes a difference.

    Cc: stable@kernel.org
    Fixes: 0baedd792713 ("KVM: x86: make Hyper-V PV TLB flush use tlb_flush_guest()")
    Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
    Message-Id: <20211209102937.584397-1-vkuznets@redhat.com>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/x86/include/asm/kvm_host.h | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/arch/x86/include/asm/kvm_host.h
    +++ b/arch/x86/include/asm/kvm_host.h
    @@ -85,7 +85,7 @@
    KVM_ARCH_REQ_FLAGS(25, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
    #define KVM_REQ_TLB_FLUSH_CURRENT KVM_ARCH_REQ(26)
    #define KVM_REQ_TLB_FLUSH_GUEST \
    - KVM_ARCH_REQ_FLAGS(27, KVM_REQUEST_NO_WAKEUP)
    + KVM_ARCH_REQ_FLAGS(27, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
    #define KVM_REQ_APF_READY KVM_ARCH_REQ(28)
    #define KVM_REQ_MSR_FILTER_CHANGED KVM_ARCH_REQ(29)


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