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    Subject[PATCH 5.0 050/115] KVM: x86: Dont clear EFER during SMM transitions for 32-bit vCPU
    Date
    From: Sean Christopherson <sean.j.christopherson@intel.com>

    commit 8f4dc2e77cdfaf7e644ef29693fa229db29ee1de upstream.

    Neither AMD nor Intel CPUs have an EFER field in the legacy SMRAM save
    state area, i.e. don't save/restore EFER across SMM transitions. KVM
    somewhat models this, e.g. doesn't clear EFER on entry to SMM if the
    guest doesn't support long mode. But during RSM, KVM unconditionally
    clears EFER so that it can get back to pure 32-bit mode in order to
    start loading CRs with their actual non-SMM values.

    Clear EFER only when it will be written when loading the non-SMM state
    so as to preserve bits that can theoretically be set on 32-bit vCPUs,
    e.g. KVM always emulates EFER_SCE.

    And because CR4.PAE is cleared only to play nice with EFER, wrap that
    code in the long mode check as well. Note, this may result in a
    compiler warning about cr4 being consumed uninitialized. Re-read CR4
    even though it's technically unnecessary, as doing so allows for more
    readable code and RSM emulation is not a performance critical path.

    Fixes: 660a5d517aaab ("KVM: x86: save/load state on SMM switch")
    Cc: stable@vger.kernel.org
    Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/kvm/emulate.c | 23 ++++++++++++-----------
    1 file changed, 12 insertions(+), 11 deletions(-)

    --- a/arch/x86/kvm/emulate.c
    +++ b/arch/x86/kvm/emulate.c
    @@ -2575,15 +2575,13 @@ static int em_rsm(struct x86_emulate_ctx
    * CR0/CR3/CR4/EFER. It's all a bit more complicated if the vCPU
    * supports long mode.
    */
    - cr4 = ctxt->ops->get_cr(ctxt, 4);
    if (emulator_has_longmode(ctxt)) {
    struct desc_struct cs_desc;

    /* Zero CR4.PCIDE before CR0.PG. */
    - if (cr4 & X86_CR4_PCIDE) {
    + cr4 = ctxt->ops->get_cr(ctxt, 4);
    + if (cr4 & X86_CR4_PCIDE)
    ctxt->ops->set_cr(ctxt, 4, cr4 & ~X86_CR4_PCIDE);
    - cr4 &= ~X86_CR4_PCIDE;
    - }

    /* A 32-bit code segment is required to clear EFER.LMA. */
    memset(&cs_desc, 0, sizeof(cs_desc));
    @@ -2597,13 +2595,16 @@ static int em_rsm(struct x86_emulate_ctx
    if (cr0 & X86_CR0_PE)
    ctxt->ops->set_cr(ctxt, 0, cr0 & ~(X86_CR0_PG | X86_CR0_PE));

    - /* Now clear CR4.PAE (which must be done before clearing EFER.LME). */
    - if (cr4 & X86_CR4_PAE)
    - ctxt->ops->set_cr(ctxt, 4, cr4 & ~X86_CR4_PAE);
    -
    - /* And finally go back to 32-bit mode. */
    - efer = 0;
    - ctxt->ops->set_msr(ctxt, MSR_EFER, efer);
    + if (emulator_has_longmode(ctxt)) {
    + /* Clear CR4.PAE before clearing EFER.LME. */
    + cr4 = ctxt->ops->get_cr(ctxt, 4);
    + if (cr4 & X86_CR4_PAE)
    + ctxt->ops->set_cr(ctxt, 4, cr4 & ~X86_CR4_PAE);
    +
    + /* And finally go back to 32-bit mode. */
    + efer = 0;
    + ctxt->ops->set_msr(ctxt, MSR_EFER, efer);
    + }

    smbase = ctxt->ops->get_smbase(ctxt);


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