Messages in this thread | | | Subject | Re: [RFC v2-fix-v4 1/1] x86/tdx: Skip WBINVD instruction for TDX guest | From | Andy Lutomirski <> | Date | Tue, 8 Jun 2021 21:02:34 -0700 |
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On 6/8/21 8:40 PM, Dan Williams wrote: > On Tue, Jun 8, 2021 at 6:10 PM Kuppuswamy Sathyanarayanan > <sathyanarayanan.kuppuswamy@linux.intel.com> wrote: >> >> Current TDX spec does not have support to emulate the WBINVD >> instruction. If any feature that uses WBINVD is enabled/used >> in TDX guest, it will lead to un-handled #VE exception, which >> will be handled as #GP fault. >> >> ACPI drivers also uses WBINVD instruction for cache flushes in >> reboot or shutdown code path. Since TDX guest has requirement >> to support shutdown feature, skip WBINVD instruction usage >> in ACPI drivers for TDX guest. > > This sounds awkward... > >> Since cache is always coherent in TDX guests, making wbinvd as > > This is incorrect, ACPI cache flushing is not about I/O or CPU coherency... > >> noop should not cause any issues in above mentioned code path. > > ..."should" is a famous last word... > >> The end-behavior is the same as KVM guest (treat as noops). > > ..."KVM gets away with it" is not a justification that TDX can stand > on otherwise we would not be here fixing up ACPICA properly. > > How about: > > "TDX guests use standard ACPI mechanisms to signal sleep state entry > (including reboot) to the host. The ACPI specification mandates WBINVD > on any sleep state entry with the expectation that the platform is > only responsible for maintaining the state of memory over sleep > states, not preserving dirty data in any CPU caches. ACPI cache > flushing requirements pre-date the advent of virtualization. Given TDX > guest sleep state entry does not affect any host power rails it is not > required to flush caches. The host is responsible for maintaining > cache state over its own bare metal sleep state transitions that > power-off the cache. If the host fails to manage caches over its sleep > state transitions the guest..." >
I like this description, but shouldn't the logic be:
if (!CPUID has hypervisor bit set) wbinvd();
As far as I know, most hypervisors will turn WBINVD into a noop and, even if they don't, it seems to be that something must be really quite wrong for a guest to need to WBINVD for ACPI purposes.
-Andy
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