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Message-ID: <20241028053431.3439593-5-nikunj@amd.com>
Date: Mon, 28 Oct 2024 11:04:22 +0530
From: Nikunj A Dadhania <nikunj@....com>
To: <linux-kernel@...r.kernel.org>, <thomas.lendacky@....com>, <bp@...en8.de>,
<x86@...nel.org>, <kvm@...r.kernel.org>
CC: <mingo@...hat.com>, <tglx@...utronix.de>, <dave.hansen@...ux.intel.com>,
<pgonda@...gle.com>, <seanjc@...gle.com>, <pbonzini@...hat.com>,
<nikunj@....com>
Subject: [PATCH v14 04/13] x86/sev: Change TSC MSR behavior for Secure TSC enabled guests
Secure TSC enabled guests should not write to 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 reads of MSR_IA32_TSC should return the result of the RDTSC
instruction.
Signed-off-by: Nikunj A Dadhania <nikunj@....com>
Reviewed-by: Tom Lendacky <thomas.lendacky@....com>
Tested-by: Peter Gonda <pgonda@...gle.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 88cae62382c2..585022b26028 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 && (sev_status & MSR_AMD64_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
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