linux-kernel - Re: [PATCH v14 04/13] x86/sev: Change TSC MSR behavior for Secure TSC enabled guests

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Message-ID: <20241101164053.GLZyUElVm8I22ZZjor@fat_crate.local>
Date: Fri, 1 Nov 2024 17:40:53 +0100
From: Borislav Petkov <bp@...en8.de>
To: Nikunj A Dadhania <nikunj@....com>
Cc: linux-kernel@...r.kernel.org, thomas.lendacky@....com, x86@...nel.org,
	kvm@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
	dave.hansen@...ux.intel.com, pgonda@...gle.com, seanjc@...gle.com,
	pbonzini@...hat.com
Subject: Re: [PATCH v14 04/13] x86/sev: Change TSC MSR behavior for Secure
 TSC enabled guests

On Mon, Oct 28, 2024 at 11:04:22AM +0530, Nikunj A Dadhania wrote:
> +	/*
> +	 * TSC related accesses should not exit to the hypervisor when a
> +	 * guest is executing with SecureTSC enabled, so special handling
> +	 * is required for accesses of MSR_IA32_TSC:
> +	 *
> +	 * Writes: Writing to MSR_IA32_TSC can cause subsequent reads
> +	 *         of the TSC to return undefined values, so ignore all
> +	 *         writes.
> +	 * Reads:  Reads of MSR_IA32_TSC should return the current TSC
> +	 *         value, use the value returned by RDTSC.
> +	 */
> +	if (regs->cx == MSR_IA32_TSC && (sev_status & MSR_AMD64_SNP_SECURE_TSC)) {
> +		u64 tsc;
> +
> +		if (exit_info_1)
> +			return ES_OK;
> +
> +		tsc = rdtsc();

rdtsc_ordered() I guess.

> +		regs->ax = UINT_MAX & tsc;
> +		regs->dx = UINT_MAX & (tsc >> 32);
> +
> +		return ES_OK;
> +	}
> +

All that you're adding - put that in a __vc_handle_msr_tsc() helper so that it
doesn't distract from the function's flow.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette