linux-kernel - Re: [PATCH v2] x86: Skip WBINVD instruction for VM guest

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Date:   Fri, 3 Dec 2021 16:20:34 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        "H . Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org
Subject: Re: [PATCH v2] x86: Skip WBINVD instruction for VM guest

On 12/3/21 3:49 PM, Kirill A. Shutemov wrote:
> -	ACPI_FLUSH_CPU_CACHE();
> +	if (acpi_state >= ACPI_STATE_S1 && acpi_state <= ACPI_STATE_S3)
> +		ACPI_FLUSH_CPU_CACHE();

It's a bit of a bummer that this per-sleep-state logic has to be
repeated so many time.

If you pass acpi_state into ACPI_FLUSH_CPU_CACHE() can you centralize
the set of places where that knowledge about which sleep states require
flushing?

> TDX doesn't support these S- and C-states. TDX is only supports S0 and S5.

This makes me a bit nervous.  Is this "the first TDX implementation
supports..." or "the TDX architecture *prohibits* supporting S1 (or
whatever"?

I really think we need some kind of architecture guarantee.  Without
that, we risk breaking things if someone at our employer simply changes
their mind.

The:

> #define ACPI_FLUSH_CPU_CACHE_PHYS()     \
>         if (!cpu_feature_enabled(XXX))	\
>         	wbinvd();               \  

does seem simpler and less error-prone than this, though.