On 10/23/2024 8:55 AM, Xiaoyao Li wrote: > On 10/21/2024 1:51 PM, Nikunj A Dadhania wrote: >> Secure TSC enabled guests should not write to MSR_IA32_TSC(10H) register as >> the subsequent TSC value reads are undefined. MSR_IA32_TSC read/write >> accesses should not exit to the hypervisor for such guests. >> >> Accesses to MSR_IA32_TSC needs special handling in the #VC handler for the >> guests with Secure TSC enabled. Writes to MSR_IA32_TSC should be ignored, >> and reads of MSR_IA32_TSC should return the result of the RDTSC >> instruction. >> >> Signed-off-by: Nikunj A Dadhania <nikunj@xxxxxxx> >> Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx> >> Tested-by: Peter Gonda <pgonda@xxxxxxxxxx> >> --- >> arch/x86/coco/sev/core.c | 24 ++++++++++++++++++++++++ >> 1 file changed, 24 insertions(+) >> >> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c >> index 965209067f03..2ad7773458c0 100644 >> --- a/arch/x86/coco/sev/core.c >> +++ b/arch/x86/coco/sev/core.c >> @@ -1308,6 +1308,30 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt) >> return ES_OK; >> } >> + /* >> + * TSC related accesses should not exit to the hypervisor when a >> + * guest is executing with SecureTSC enabled, so special handling >> + * is required for accesses of MSR_IA32_TSC: >> + * >> + * Writes: Writing to MSR_IA32_TSC can cause subsequent reads >> + * of the TSC to return undefined values, so ignore all >> + * writes. >> + * Reads: Reads of MSR_IA32_TSC should return the current TSC >> + * value, use the value returned by RDTSC. >> + */ > > Why doesn't handle it by returning ES_VMM_ERROR when hypervisor intercepts > RD/WR of MSR_IA32_TSC? With SECURE_TSC enabled, it seems not need to be > intercepted. ES_VMM_ERROR will terminate the guest, which is not the expected behaviour. As documented, writes to the MSR is ignored and reads are done using RDTSC. > I think the reason is that SNP guest relies on interception to do the ignore > behavior for WRMSR in #VC handler because the writing leads to undefined > result. For legacy and secure guests MSR_IA32_TSC is always intercepted(for both RD/WR). Moreover, this is a legacy MSR, RDTSC and RDTSCP is the what modern OSes should use. The idea is the catch any writes to TSC MSR and handle them gracefully. > Then the question is what if the hypervisor doesn't intercept write to > MSR_IA32_TSC in the first place? I have tried to disable interception of MSR_IA32_TSC, and writes are ignored by the HW as well. I would like to continue the current documented HW as per the APM. Regards, Nikunj
