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