Re: [PATCH] x86/tsx: clear RTM and HLE when MSR_IA32_TSX_CTRL is not supported

[Hao Peng]

On Tue, Sep 7, 2021 at 1:13 PM Pawan Gupta
<pawan.kumar.gupta@xxxxxxxxxxxxxxx> wrote:
>
> On 06.09.2021 10:46, Hao Peng wrote:
> >If hypervisor does not support MSR_IA32_TSX_CTRL, but guest supports
> >RTM and HLE features, it will affect TAA mitigation.
>
> Guests are on purpose not allowed to control TSX via MSR_IA32_TSX_CTRL,
> otherwise a malicious guest can enable TSX and attack host or other
> guests. The TAA mitigation within a guest is same as MDS i.e.
> micro-architectural buffer clear using VERW instruction. Support for
> VERW is added by the microcode update and enumerate by
> MSR_ARCH_CAP[MD_CLEAR] bit.
>
> >Signed-off-by: Peng Hao <flyingpeng@xxxxxxxxxxx>
> >---
> > arch/x86/kernel/cpu/tsx.c | 7 +++++++
> > 1 file changed, 7 insertions(+)
> >
> >diff --git a/arch/x86/kernel/cpu/tsx.c b/arch/x86/kernel/cpu/tsx.c
> >index 9c7a5f049292..5e852c14fef2 100644
> >--- a/arch/x86/kernel/cpu/tsx.c
> >+++ b/arch/x86/kernel/cpu/tsx.c
> >@@ -122,6 +122,13 @@ void __init tsx_init(void)
> >
> >        if (!tsx_ctrl_is_supported()) {
> >                tsx_ctrl_state = TSX_CTRL_NOT_SUPPORTED;
> >+
> >+               /* If hypervisor does not support MSR_IA32_TSX_CTRL emulation,
> >+                * but guest supports RTM and HLE features, it will affect TAA
> >+                * （tsx_async_abort）mitigation.
> >+                */
> >+               setup_clear_cpu_cap(X86_FEATURE_RTM);
> >+               setup_clear_cpu_cap(X86_FEATURE_HLE);
>
> This is not correct. TSX feature can exist without TSX_CTRL MSR.
> Moreover, clearing the cached bits with setup_clear_cpu_cap() doesn't
> disable the TSX feature in CPU.
>
After applying this patch, the output of
/sys/devices/system/cpu/vulnerabilities/tsx_async_abort
becomes “Mitigation: TSX disabled”.Do you mean that tsx is still
enabled in this case in guest?
I made a mistake in the description before. This problem occurred
under the qemu -cpu Icelake-server .
When I debug this problem to -cpu host, the guest can see taa-no.
Thanks.
> Thanks,
> Pawan