linux-kernel - Re: [PATCHv2] x86/tdx: Mark TSC reliable

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Date:   Tue, 22 Aug 2023 16:46:56 -0700
From:   Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Elena Reshetova <elena.reshetova@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>, x86@...nel.org,
        linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2] x86/tdx: Mark TSC reliable



On 8/22/2023 4:18 PM, Kirill A. Shutemov wrote:
> In x86 virtualization environments, including TDX, RDTSC instruction is
> handled without causing a VM exit, resulting in minimal overhead and
> jitters. On the other hand, other clock sources (such as HPET, ACPI
> timer, APIC, etc.) necessitate VM exits to implement, resulting in more
> fluctuating measurements compared to TSC. Thus, those clock sources are
> not effective for calibrating TSC.
> 
> In TD guests, TSC is virtualized by the TDX module, which ensures:
> 
>   - Virtual TSC values are consistent among all the TD’s VCPUs;
>   - Monotonously incrementing for any single VCPU;
>   - The frequency is determined by TD configuration. The host TSC is
>     invariant on platforms where TDX is available.
> 
> Reliable TSC is architectural guarantee for the TDX platform and it must
> work for any sane TDX implementation.
> 
> Use TSC as the only reliable clock source in TD guests, bypassing
> unstable calibration.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  v2:
>   - Slightly updated commit message;
> ---

Looks good to me.

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>

>  arch/x86/coco/tdx/tdx.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 1d6b863c42b0..1583ec64d92e 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -769,6 +769,9 @@ void __init tdx_early_init(void)
>  
>  	setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);
>  
> +	/* TSC is the only reliable clock in TDX guest */
> +	setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
> +
>  	cc_vendor = CC_VENDOR_INTEL;
>  	tdx_parse_tdinfo(&cc_mask);
>  	cc_set_mask(cc_mask);

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer