Re: [PATCH v15 09/13] tsc: Use the GUEST_TSC_FREQ MSR for discovering TSC frequency

Tom Lendacky
2024-12-16T16:31:32Z

On 12/3/24 03:00, Nikunj A Dadhania wrote:
> Calibrating the TSC frequency using the kvmclock is not correct for
> SecureTSC enabled guests. Use the platform provided TSC frequency via the
> GUEST_TSC_FREQ MSR (C001_0134h).
>
> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
> ---
> arch/x86/include/asm/sev.h | 2 ++
> arch/x86/coco/sev/core.c | 16 ++++++++++++++++
> arch/x86/kernel/tsc.c | 5 +++++
> 3 files changed, 23 insertions(+)
>
> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> index 9fd02efef08e..c4dca06b3b01 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -493,6 +493,7 @@ int snp_send_guest_request(struct snp_msg_desc *mdesc, struct snp_guest_req *req
> struct snp_guest_request_ioctl *rio);
>
> void __init snp_secure_tsc_prepare(void);
> +void __init snp_secure_tsc_init(void);
>
> #else /* !CONFIG_AMD_MEM_ENCRYPT */
>
> @@ -536,6 +537,7 @@ static inline void snp_msg_free(struct snp_msg_desc *mdesc) { }
> static inline int snp_send_guest_request(struct snp_msg_desc *mdesc, struct snp_guest_req *req,
> struct snp_guest_request_ioctl *rio) { return -ENODEV; }
> static inline void __init snp_secure_tsc_prepare(void) { }
> +static inline void __init snp_secure_tsc_init(void) { }
>
> #endif /* CONFIG_AMD_MEM_ENCRYPT */
>
> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> index 59c5e716fdd1..1bc668883058 100644
> --- a/arch/x86/coco/sev/core.c
> +++ b/arch/x86/coco/sev/core.c
> @@ -3279,3 +3279,19 @@ void __init snp_secure_tsc_prepare(void)
>
> pr_debug("SecureTSC enabled");
> }
> +
> +static unsigned long securetsc_get_tsc_khz(void)
> +{
> + unsigned long long tsc_freq_mhz;
> +
> + setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
> + rdmsrl(MSR_AMD64_GUEST_TSC_FREQ, tsc_freq_mhz);

This should never change, right? Can this be put in snp_secure_tsc_init()
and just return a saved value that is already in khz form? No reason to
perform the MSR access and multiplication every time.

Thanks,
Tom

> +
> + return (unsigned long)(tsc_freq_mhz * 1000);
> +}
> +
> +void __init snp_secure_tsc_init(void)
> +{
> + x86_platform.calibrate_cpu = securetsc_get_tsc_khz;
> + x86_platform.calibrate_tsc = securetsc_get_tsc_khz;
> +}
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 67aeaba4ba9c..c0eef924b84e 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -30,6 +30,7 @@
> #include <asm/i8259.h>
> #include <asm/topology.h>
> #include <asm/uv/uv.h>
> +#include <asm/sev.h>
>
> unsigned int __read_mostly cpu_khz; /* TSC clocks / usec, not used here */
> EXPORT_SYMBOL(cpu_khz);
> @@ -1515,6 +1516,10 @@ void __init tsc_early_init(void)
> /* Don't change UV TSC multi-chassis synchronization */
> if (is_early_uv_system())
> return;
> +
> + if (cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC))
> + snp_secure_tsc_init();
> +
> if (!determine_cpu_tsc_frequencies(true))
> return;
> tsc_enable_sched_clock();