Re: [PATCH v16 06/13] x86/sev: Change TSC MSR behavior for Secure TSC enabled guests

Tom Lendacky
2025-01-07T20:10:05Z

On 1/6/25 06:46, Nikunj A Dadhania wrote:
> Secure TSC enabled guests should not write to the MSR_IA32_TSC(10H)
> register as the subsequent TSC value reads are undefined. For AMD platform,
> MSR_IA32_TSC is intercepted by the hypervisor. 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 flagged once with a warning, and reads of MSR_IA32_TSC should return
> the result of the RDTSC instruction.
>
> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

> ---
> arch/x86/coco/sev/core.c | 39 ++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 38 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> index 00a0ac3baab7..f49d3e97e170 100644
> --- a/arch/x86/coco/sev/core.c
> +++ b/arch/x86/coco/sev/core.c
> @@ -1428,6 +1428,34 @@ static enum es_result __vc_handle_msr_caa(struct pt_regs *regs, bool write)
> return ES_OK;
> }
>
> +/*
> + * TSC related accesses should not exit to the hypervisor when a guest is
> + * executing with Secure TSC enabled, so special handling is required for
> + * accesses of MSR_IA32_TSC.
> + */
> +static enum es_result __vc_handle_secure_tsc_msrs(struct pt_regs *regs, bool write)
> +{
> + u64 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_ordered().
> + */
> + if (write) {
> + WARN_ONCE(1, "TSC MSR writes are verboten!\n");
> + return ES_OK;
> + }
> +
> + tsc = rdtsc_ordered();
> + regs->ax = lower_32_bits(tsc);
> + regs->dx = upper_32_bits(tsc);
> +
> + return ES_OK;
> +}
> +
> static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
> {
> struct pt_regs *regs = ctxt->regs;
> @@ -1437,8 +1465,17 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
> /* Is it a WRMSR? */
> write = ctxt->insn.opcode.bytes[1] == 0x30;
>
> - if (regs->cx == MSR_SVSM_CAA)
> + switch (regs->cx) {
> + case MSR_SVSM_CAA:
> return __vc_handle_msr_caa(regs, write);
> + case MSR_IA32_TSC:
> + if (sev_status & MSR_AMD64_SNP_SECURE_TSC)
> + return __vc_handle_secure_tsc_msrs(regs, write);
> + else
> + break;
> + default:
> + break;
> + }
>
> ghcb_set_rcx(ghcb, regs->cx);
> if (write) {