Re: [PATCH v14 09/13] tsc: Use the GUEST_TSC_FREQ MSR for discovering TSC frequency

["Nikunj A. Dadhania" <nikunj@amd.com>]

On 10/29/2024 8:32 AM, Xiaoyao Li wrote:
> On 10/28/2024 1:34 PM, 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 d27c4e0f9f57..9ee63ddd0d90 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 snp_secure_tsc_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 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 140759fafe0c..0be9496b8dea 100644
>> --- a/arch/x86/coco/sev/core.c
>> +++ b/arch/x86/coco/sev/core.c
>> @@ -3064,3 +3064,19 @@ 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);
>> +
>> +    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 dfe6847fd99e..730cbbd4554e 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))
>> +        snp_secure_tsc_init();
> 
> IMHO, it isn't the good place to call snp_secure_tsc_init() to update the callbacks here.
> 
> It's better to be called in some snp init functions.

As part of setup_arch(), init_hypervisor_platform() gets called and all the PV clocks 
are registered and initialized as part of init_platform callback. Once the hypervisor 
platform is initialized, tsc_early_init() is called. SEV SNP guest can be running on
any hypervisor, so the call back needs to be updated either in tsc_early_init() or
init_hypervisor_platform(), as the change is TSC related, I have updated it here.

> 
>>       if (!determine_cpu_tsc_frequencies(true))
>>           return;
>>       tsc_enable_sched_clock();
> 

Regards,
Nikunj