linux-kernel - Re: [PATCH v11 0/8] KVM: SVM: Enable Secure TSC for SEV-SNP

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Message-ID: <afcf9a0b-7450-4df7-a21b-80b56264fc15@amd.com>
Date: Wed, 20 Aug 2025 14:18:18 +0530
From: "Nikunj A. Dadhania" <nikunj@....com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
	<pbonzini@...hat.com>
CC: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Thomas Lendacky
	<thomas.lendacky@....com>, Michael Roth <michael.roth@....com>, "Borislav
 Petkov" <bp@...en8.de>, Vaishali Thakkar <vaishali.thakkar@...e.com>, "Ketan
 Chaturvedi" <Ketan.Chaturvedi@....com>, Kai Huang <kai.huang@...el.com>
Subject: Re: [PATCH v11 0/8] KVM: SVM: Enable Secure TSC for SEV-SNP



On 8/20/2025 5:18 AM, Sean Christopherson wrote:
> This is a combination of Nikunk's series to enable secure TSC support and to
                       
                          *Nikunj's 😊 (close though!)

> fix the GHCB version issues, along with some code refactorings to move SEV+
> setup code into sev.c (we've managed to grow something like 4 flows that all
> do more or less the same thing).
> 
> Note, I haven't tested SNP functionality in any way.

Tested SNP with and without Secure TSC, guest works as expected.

> 
> v11:
>  - Shuffle code around so that snp_is_secure_tsc_enabled() doesn't need to
>    be exposed outside of sev.c.
>  - Explicitly modify the intercept for MSR_AMD64_GUEST_TSC_FREQ (paranoia is
>    cheap in this case).
>  - Trim the changelog for the GHCB version enforcement patch.

>  - Continue on with snp_launch_start() if default_tsc_khz is '0'.  AFAICT,
>    continuing on doesn't put the host at (any moer) risk. [Kai]

If I hack default_tsc_khz  as '0', SNP guest kernel with SecureTSC spits out
couple of warnings and finally panics:
 
 Persistent clock returned invalid value
 ------------[ cut here ]------------
 Missing cycle counter and fallback timer; RNG entropy collection will consequently suffer.
 WARNING: CPU: 0 PID: 0 at drivers/char/random.c:931 random_init+0xe7/0xf0
 RIP: 0010:random_init+0xe7/0xf0
 Call Trace:
  <TASK>
  start_kernel+0x5e9/0xb80
  x86_64_start_reservations+0x18/0x30
  x86_64_start_kernel+0xf5/0x140
  common_startup_64+0x13e/0x141
  </TASK>

...

 WARNING: CPU: 0 PID: 0 at arch/x86/kernel/tsc.c:1464 determine_cpu_tsc_frequencies+0x118/0x120
 CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Tainted: G        W           6.17.0-rc2-stsc+ #1063 PREEMPT(voluntary)
 RIP: 0010:determine_cpu_tsc_frequencies+0x118/0x120
 Call Trace:
  <TASK>
  tsc_init+0x2ba/0x430
  x86_late_time_init+0x29/0x40
  start_kernel+0x70b/0xb80
  x86_64_start_reservations+0x18/0x30
  x86_64_start_kernel+0xf5/0x140
  common_startup_64+0x13e/0x141
  </TASK>

...

 Oops: divide error: 0000 [#1] SMP NOPTI
 CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Tainted: G        W           6.17.0-rc2-stsc+ #1063 PREEMPT(voluntary)
 Tainted: [W]=WARN
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown 02/02/2022
 RIP: 0010:pit_hpet_ptimer_calibrate_cpu+0x1be/0x410
 Call Trace:
  <TASK>
  determine_cpu_tsc_frequencies+0xc1/0x120
  tsc_init+0x2ba/0x430
  x86_late_time_init+0x29/0x40
  start_kernel+0x70b/0xb80
  x86_64_start_reservations+0x18/0x30
  x86_64_start_kernel+0xf5/0x140
  common_startup_64+0x13e/0x141
  </TASK>

Regards,
Nikunj