[PATCH v13 04/13] x86/sev: Change TSC MSR behavior for Secure TSC enabled guests

[Nikunj A Dadhania <nikunj@amd.com>]

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@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Tested-by: Peter Gonda <pgonda@google.com>
---
 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.
+	 */
+	if (regs->cx == MSR_IA32_TSC && cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC)) {
+		u64 tsc;
+
+		if (exit_info_1)
+			return ES_OK;
+
+		tsc = rdtsc();
+		regs->ax = UINT_MAX & tsc;
+		regs->dx = UINT_MAX & (tsc >> 32);
+
+		return ES_OK;
+	}
+
 	ghcb_set_rcx(ghcb, regs->cx);
 	if (exit_info_1) {
 		ghcb_set_rax(ghcb, regs->ax);
-- 
2.34.1