Re: [PATCH v12 14/19] tsc: Use the GUEST_TSC_FREQ MSR for discovering TSC frequency

[Tom Lendacky]

On 10/9/24 04:28, 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@xxxxxxx>
> ---
>  arch/x86/include/asm/msr-index.h |  1 +
>  arch/x86/include/asm/sev.h       |  2 ++
>  arch/x86/coco/sev/core.c         | 16 ++++++++++++++++
>  arch/x86/kernel/tsc.c            |  5 +++++
>  4 files changed, 24 insertions(+)
> 
> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> index 3ae84c3b8e6d..233be13cc21f 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -608,6 +608,7 @@
>  #define MSR_AMD_PERF_CTL		0xc0010062
>  #define MSR_AMD_PERF_STATUS		0xc0010063
>  #define MSR_AMD_PSTATE_DEF_BASE		0xc0010064
> +#define MSR_AMD64_GUEST_TSC_FREQ	0xc0010134
>  #define MSR_AMD64_OSVW_ID_LENGTH	0xc0010140
>  #define MSR_AMD64_OSVW_STATUS		0xc0010141
>  #define MSR_AMD_PPIN_CTL		0xc00102f0
> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> index 9169b18eeb78..34f7b9fc363b 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -536,6 +536,7 @@ static inline int handle_guest_request(struct snp_msg_desc *mdesc, u64 exit_code
>  }
>  
>  void __init snp_secure_tsc_prepare(void);
> +void __init securetsc_init(void);
>  
>  #else	/* !CONFIG_AMD_MEM_ENCRYPT */
>  
> @@ -584,6 +585,7 @@ static inline int handle_guest_request(struct snp_msg_desc *mdesc, u64 exit_code
>  				       u32 resp_sz) { return -ENODEV; }
>  
>  static inline void __init snp_secure_tsc_prepare(void) { }
> +static inline void __init securetsc_init(void) { }
>  
>  #endif	/* CONFIG_AMD_MEM_ENCRYPT */
>  
> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> index 5f555f905fad..ef0def203b3f 100644
> --- a/arch/x86/coco/sev/core.c
> +++ b/arch/x86/coco/sev/core.c
> @@ -3100,3 +3100,19 @@ 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);

So this MSR can be intercepted by the hypervisor. You'll need to add
code in the #VC handler that checks if an MSR access is for
MSR_AMD64_GUEST_TSC_FREQ and Secure TSC is active, then the hypervisor
is not cooperating and you should terminate the guest.

Thanks,
Tom

> +
> +	return (unsigned long)(tsc_freq_mhz * 1000);
> +}
> +
> +void __init securetsc_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 dfe6847fd99e..c83f1091bb4f 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);
> @@ -1514,6 +1515,10 @@ 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))
> +		securetsc_init();
> +
>  	if (!determine_cpu_tsc_frequencies(true))
>  		return;
>  	tsc_enable_sched_clock();