linux-kernel - Re: [PATCHv6 07/30] x86/traps: Add #VE support for TDX guest

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Message-ID: <20220318153454.7ciwicqfghiwnv2t@black.fi.intel.com>
Date:   Fri, 18 Mar 2022 18:34:54 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...el.com, luto@...nel.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,
        thomas.lendacky@....com, brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCHv6 07/30] x86/traps: Add #VE support for TDX guest

On Fri, Mar 18, 2022 at 03:19:34PM +0100, Thomas Gleixner wrote:
> On Thu, Mar 17 2022 at 21:21, Peter Zijlstra wrote:
> > On Thu, Mar 17, 2022 at 08:33:54PM +0300, Kirill A. Shutemov wrote:
> >>  - in NMI entry code (asm_exc_nmi()) before NMI nesting is safe:
> >>    + for NMI from user mode, before switched to thread stack
> >>    + for NMI from kernel, up to end_repead_nmi
> >> 
> >> After that points #VE is safe.
> >
> > In what way is it guaranteed that #VE isn't raised in those places? What
> > does an auditor / future coder looking to changes things, need to
> > consider to keep this so.
> >
> > From vague memories #VE can be raised on any memop, loading the stack
> > address in the syscall-gap is a memop. What makes that special? Can we
> > get a comment _there_ to explain how this is safe such that we can keep
> > it so?
> >
> > Same for the NMI path I suppose.
> 
> #VE is raised by HLT, CPUID, I/O-Port access, MSR read/write, EPT violations
> 
> So in the hairy places:
> 
>  - HLT:       No business
>  - I/O Ports: That would be outright stupid to use
> 
>  - CPUID:     Should never be used - Emphasis on should :)
>  - MSRs:      Same as CPUID
> 
>  - EPT:       Well....

EPT violation may result in #VE only on shared memory. If entry code
touches shared memory we have a bigger problem than syscall gap.

-- 
 Kirill A. Shutemov