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Message-ID: <ff5d23fa-12c6-47bb-8309-b19d39875827@intel.com>
Date: Tue, 29 Oct 2024 17:15:11 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: "Nikunj A. Dadhania" <nikunj@....com>, linux-kernel@...r.kernel.org,
thomas.lendacky@....com, bp@...en8.de, x86@...nel.org, kvm@...r.kernel.org
Cc: mingo@...hat.com, tglx@...utronix.de, dave.hansen@...ux.intel.com,
pgonda@...gle.com, seanjc@...gle.com, pbonzini@...hat.com
Subject: Re: [PATCH v14 09/13] tsc: Use the GUEST_TSC_FREQ MSR for discovering
TSC frequency
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@....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
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