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

Nikunj A Dadhania
2025-01-06T12:47:31Z

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>
---
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) {
--
2.34.1