Re: [PATCH v2 07/11] x86/sev: Change TSC MSR behavior for Secure TSC enabled guests

[Tom Lendacky <thomas.lendacky@amd.com>]

On 3/26/23 09:46, Nikunj A Dadhania wrote:
> Secure TSC enabled guests should not write MSR_IA32_TSC(10H) register
> as the subsequent TSC value reads are undefined. MSR_IA32_TSC related
> 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>

> ---
>   arch/x86/kernel/sev.c | 24 ++++++++++++++++++++++++
>   1 file changed, 24 insertions(+)
> 
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index c5ca97aab8c1..3750e545d688 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -1642,6 +1642,30 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
>   	/* Is it a WRMSR? */
>   	exit_info_1 = (ctxt->insn.opcode.bytes[1] == 0x30) ? 1 : 0;
>   
> +	/*
> +	 * 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);