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

[Nikunj A Dadhania <nikunj@xxxxxxx>]

On 12/16/2024 10:01 PM, Tom Lendacky wrote:
> 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@xxxxxxx>
>> ---
>>  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.

This happens a couple of times during the boot, so I think this does not
have much overhead. Something like below ?

diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index c7870294a957..69b65c4c850c 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -103,6 +103,7 @@ static u64 secrets_pa __ro_after_init;
  */
 static u64 snp_tsc_scale __ro_after_init;
 static u64 snp_tsc_offset __ro_after_init;
+static u64 snp_tsc_freq_khz __ro_after_init;
 
 /* #VC handler runtime per-CPU data */
 struct sev_es_runtime_data {
@@ -3282,16 +3283,18 @@ void __init snp_secure_tsc_prepare(void)
 
 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);
 
-	return (unsigned long)(tsc_freq_mhz * 1000);
+	return snp_tsc_freq_khz;
 }
 
 void __init snp_secure_tsc_init(void)
 {
+	unsigned long long tsc_freq_mhz;
+
+	rdmsrl(MSR_AMD64_GUEST_TSC_FREQ, tsc_freq_mhz);
+	snp_tsc_freq_khz = (unsigned long)(tsc_freq_mhz * 1000);
+
 	x86_platform.calibrate_cpu = securetsc_get_tsc_khz;
 	x86_platform.calibrate_tsc = securetsc_get_tsc_khz;
 }


---

Regards
Nikunj