linux-kernel - Re: [PATCH v11 19/20] x86/kvmclock: Skip kvmclock when Secure TSC is available

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Message-ID: <4cc88621-d548-d3a1-d667-13586b7bfea8@amd.com>
Date: Fri, 20 Sep 2024 14:24:10 +0530
From: "Nikunj A. Dadhania" <nikunj@....com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: linux-kernel@...r.kernel.org, thomas.lendacky@....com, bp@...en8.de,
 x86@...nel.org, kvm@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
 dave.hansen@...ux.intel.com, pgonda@...gle.com, pbonzini@...hat.com
Subject: Re: [PATCH v11 19/20] x86/kvmclock: Skip kvmclock when Secure TSC is
 available

On 9/20/2024 12:51 PM, Sean Christopherson wrote:
> On Fri, Sep 20, 2024, Nikunj A. Dadhania wrote:
>> On 9/18/2024 5:37 PM, Sean Christopherson wrote:
>>> On Mon, Sep 16, 2024, Nikunj A. Dadhania wrote:
>>>> On 9/13/2024 11:00 PM, Sean Christopherson wrote:
>>>>>> Signed-off-by: Nikunj A Dadhania <nikunj@....com>
>>>>>> Tested-by: Peter Gonda <pgonda@...gle.com>
>>>>>> ---
>>>>>>  arch/x86/kernel/kvmclock.c | 2 +-
>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
>>>>>> index 5b2c15214a6b..3d03b4c937b9 100644
>>>>>> --- a/arch/x86/kernel/kvmclock.c
>>>>>> +++ b/arch/x86/kernel/kvmclock.c
>>>>>> @@ -289,7 +289,7 @@ void __init kvmclock_init(void)
>>>>>>  {
>>>>>>  	u8 flags;
>>>>>>  
>>>>>> -	if (!kvm_para_available() || !kvmclock)
>>>>>> +	if (!kvm_para_available() || !kvmclock || cc_platform_has(CC_ATTR_GUEST_SECURE_TSC))
>>>>>
>>>>> I would much prefer we solve the kvmclock vs. TSC fight in a generic way.  Unless
>>>>> I've missed something, the fact that the TSC is more trusted in the SNP/TDX world
>>>>> is simply what's forcing the issue, but it's not actually the reason why Linux
>>>>> should prefer the TSC over kvmclock.  The underlying reason is that platforms that
>>>>> support SNP/TDX are guaranteed to have a stable, always running TSC, i.e. that the
>>>>> TSC is a superior timesource purely from a functionality perspective.  That it's
>>>>> more secure is icing on the cake.
>>>>
>>>> Are you suggesting that whenever the guest is either SNP or TDX, kvmclock
>>>> should be disabled assuming that timesource is stable and always running?
>>>
>>> No, I'm saying that the guest should prefer the raw TSC over kvmclock if the TSC
>>> is stable, irrespective of SNP or TDX.  This is effectively already done for the
>>> timekeeping base (see commit 7539b174aef4 ("x86: kvmguest: use TSC clocksource if
>>> invariant TSC is exposed")), but the scheduler still uses kvmclock thanks to the
>>> kvm_sched_clock_init() code.
>>
>> The kvm-clock and tsc-early both are having the rating of 299. As they are of
>> same rating, kvm-clock is being picked up first.
>>
>> Is it fine to drop the clock rating of kvmclock to 298 ? With this tsc-early will
>> be picked up instead.
> 
> IMO, it's ugly, but that's a problem with the rating system inasmuch as anything.
>
> But the kernel will still be using kvmclock for the scheduler clock, which is
> undesirable.

Agree, kvm_sched_clock_init() is still being called. The above hunk was to use
tsc-early/tsc as the clocksource and not kvm-clock.

Regards,
Nikunj