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

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 10/29/2024 11:56 AM, Nikunj A. Dadhania wrote:
> 
> 
> 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.

I think it might be due to

1. it lacks a central place for SNP related stuff, like tdx_early_init()
2. even we have some place of 1), the callbacks will be overwrote in 
init_hypervisor_platform() by specific PV ops.

However, I don't think it's good practice to update it tsc.c. The reason 
why callback is used is that arch/hypervisor specific code can implement
and overwrite with it's own implementation in its own file.

Back to your case, I think a central snp init function would be helpful, 
and we can introduce a new flag to skip the overwrite of tsc/cpu 
calibration for hypervisor when the flag is set.


>>
>>>        if (!determine_cpu_tsc_frequencies(true))
>>>            return;
>>>        tsc_enable_sched_clock();
>>
> 
> Regards,
> Nikunj