linux-kernel - Re: [PATCHv2 28/29] x86/tdx: Warn about unexpected WBINVD

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Message-ID: <20220204213557.n2cirp5k4ryv754q@black.fi.intel.com>
Date:   Sat, 5 Feb 2022 00:35:57 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     mingo@...hat.com, bp@...en8.de, dave.hansen@...el.com,
        luto@...nel.org, peterz@...radead.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 28/29] x86/tdx: Warn about unexpected WBINVD

On Wed, Feb 02, 2022 at 02:46:17AM +0100, Thomas Gleixner wrote:
> On Mon, Jan 24 2022 at 18:02, Kirill A. Shutemov wrote:
> 
> > WBINVD causes #VE in TDX guests. There's no reliable way to emulate it.
> > The kernel can ask for VMM assistance, but VMM is untrusted and can ignore
> > the request.
> >
> > Fortunately, there is no use case for WBINVD inside TDX guests.
> 
> If there is not usecase, then why
> 
> > Warn about any unexpected WBINVD.
> 
> instead of terminating the whole thing?
> 
> I'm tired of the "let us emit a warning in the hope it gets fixed'
> thinking.

I probably misunderstood what you meant in the previous WBINVD thread[1] by:

	Then you have the #VE handler which just acts on any other wbinvd
	invocation via warn, panic, whatever, no?

I went the warn path because I think it is consistent with BUG() vs. WARN()
policy: "Use WARN() and WARN_ON() instead, and handle the "impossible"
error condition as gracefully as possible."[2]

IMO, ignored WBINVD has less chance to lead user data loss than panic().

Anyway, I'm okay dropping the patch. It will bring us to "terminate whole
thing" solution. I just wanted to explain why the patch is present in the
series.

[1] https://lore.kernel.org/all/87lf126010.ffs@tglx/
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/deprecated.rst

-- 
 Kirill A. Shutemov