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
