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Message-ID: <b69b21b8-7915-4854-b5f7-5124cd4e5bbf@linux.intel.com>
Date:   Tue, 10 Oct 2023 06:53:27 -0700
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Subject: Re: [PATCH 04/13] x86/kvm: Do not try to disable kvmclock if it was
 not enabled
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To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        kexec@lists.infradead.org, linux-coco@lists.linux.dev,
        linux-kernel@vger.kernel.org
References: <20231005131402.14611-1-kirill.shutemov@linux.intel.com>
 <20231005131402.14611-5-kirill.shutemov@linux.intel.com>
From:   Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
In-Reply-To: <20231005131402.14611-5-kirill.shutemov@linux.intel.com>
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On 10/5/2023 6:13 AM, Kirill A. Shutemov wrote:
> kvm_guest_cpu_offline() tries to disable kvmclock regardless if it is
> present in the VM. It leads to write to a MSR that doesn't exist on some
> configurations, namely in TDX guest:
> 
> 	unchecked MSR access error: WRMSR to 0x12 (tried to write 0x0000000000000000)
> 	at rIP: 0xffffffff8110687c (kvmclock_disable+0x1c/0x30)
> 
> kvmclock enabling is gated by CLOCKSOURCE and CLOCKSOURCE2 KVM paravirt
> features.
> 
> Do not disable kvmclock if it was not enumerated or disabled by user
> from kernel command line.

For the above warning,  check for CLOCKSOURCE and CLOCKSOURCE2
feature is sufficient, right? Do we need to include user/command-line
disable check here?

> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Fixes: c02027b5742b ("x86/kvm: Disable kvmclock on all CPUs on shutdown")
> ---
>  arch/x86/kernel/kvmclock.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index fb8f52149be9..cba2e732e53f 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -22,7 +22,7 @@
>  #include <asm/x86_init.h>
>  #include <asm/kvmclock.h>
>  
> -static int kvmclock __initdata = 1;
> +static int kvmclock __ro_after_init = 1;
>  static int kvmclock_vsyscall __initdata = 1;
>  static int msr_kvm_system_time __ro_after_init = MSR_KVM_SYSTEM_TIME;
>  static int msr_kvm_wall_clock __ro_after_init = MSR_KVM_WALL_CLOCK;
> @@ -195,7 +195,12 @@ static void kvm_setup_secondary_clock(void)
>  
>  void kvmclock_disable(void)
>  {
> -	native_write_msr(msr_kvm_system_time, 0, 0);
> +	if (!kvm_para_available() || !kvmclock)
> +		return;
> +
> +	if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE) ||
> +	    kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE2))
> +		native_write_msr(msr_kvm_system_time, 0, 0);
>  }
>  
>  static void __init kvmclock_init_mem(void)

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer