Re: [PATCH v2] KVM: x86: Allow guests to see MSR_IA32_TSX_CTRL even if tsx=off

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On 01/02/21 17:38, Sean Christopherson wrote:
     /*
      * On TAA affected systems:
      *      - nothing to do if TSX is disabled on the host.
      *      - we emulate TSX_CTRL if present on the host.
      *      This lets the guest use VERW to clear CPU buffers.
      */

it says "nothing to do..." and then clears a
flag.  The other interpretation of "nothing to do... at runtime" is also wrong
as KVM emulates the MSR as a nop.

I guess I just find the whole comment more confusing than the code itself.

What about:


        if (!boot_cpu_has(X86_FEATURE_RTM)) {
                /*
* If RTM=0 because the kernel has disabled TSX, the host might * have TAA_NO or TSX_CTRL. Clear TAA_NO (the guest sees RTM=0
                 * and therefore knows that there cannot be TAA) but keep
* TSX_CTRL: some buggy userspaces leave it set on tsx=on hosts, * and we want to allow migrating those guests to tsx=off hosts.
                 */
                data &= ~ARCH_CAP_TAA_NO;
        } else if (!boot_cpu_has_bug(X86_BUG_TAA)) {
                data |= ARCH_CAP_TAA_NO;
        } else {
                /*
* Nothing to do here; we emulate TSX_CTRL if present on the
                 * host so the guest can choose between disabling TSX or
                 * using VERW to clear CPU buffers.
                 */
        }

Paolo




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