Messages in this thread Patch in this message | | | From | Kuppuswamy Sathyanarayanan <> | Subject | [RFC v2-fix-v3 1/1] x86/tdx: Ignore WBINVD instruction for TDX guest | Date | Wed, 26 May 2021 21:38:32 -0700 |
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Functionally only devices outside the CPU (such as DMA devices, or persistent memory for flushing) can notice the external side effects from WBINVD's cache flushing for write back mappings. One exception here is MKTME, but that is not visible outside the TDX module and not possible inside a TDX guest.
Currently TDX does not support DMA, because DMA typically needs uncached access for MMIO, and the current TDX module always sets the IgnorePAT bit, which prevents that.
Persistent memory is also currently not supported. There are some other cases that use WBINVD, such as the legacy ACPI sleeps, but these are all not supported in virtualization and there are better mechanisms inside a guest anyways. The guests usually are not aware of power management. Another code path that uses WBINVD is the MTRR driver, but EPT/virtualization always disables MTRRs so those are not needed. This all implies WBINVD is not needed with current TDX.
So handle the WBINVD instruction as nop. Currently, #VE exception handler does not include any warning for WBINVD handling because ACPI reboot code uses it. This is the same behavior as KVM. It only allows WBINVD in a guest when the guest supports VT-d (=DMA), but just handles it as a nop if it doesn't .
If TDX ever gets DMA support, or persistent memory support, or some other devices that can observe flushing side effects, a hypercall can be added to implement it similar to AMD-SEV. But current TDX does not need it.
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com> ---
Changes since RFC v2-2: * Added more details to commit log and comments to address review comments.
Changes since RFC v2: * Fixed commit log as per review comments. * Removed WARN_ONCE for WBINVD #VE support.
arch/x86/kernel/tdx.c | 7 +++++++ 1 file changed, 7 insertions(+)
diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c index da5c9cd08299..775ae090b625 100644 --- a/arch/x86/kernel/tdx.c +++ b/arch/x86/kernel/tdx.c @@ -455,6 +455,13 @@ int tdg_handle_virtualization_exception(struct pt_regs *regs, case EXIT_REASON_EPT_VIOLATION: ve->instr_len = tdg_handle_mmio(regs, ve); break; + case EXIT_REASON_WBINVD: + /* + * Non coherent DMA, persistent memory, MTRRs or + * outdated ACPI sleeps are not supported in TDX guest. + * So ignore WBINVD and treat it nop. + */ + break; case EXIT_REASON_MONITOR_INSTRUCTION: case EXIT_REASON_MWAIT_INSTRUCTION: /* -- 2.25.1
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