Date:   Wed, 29 Dec 2021 12:29:51 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     tglx@linutronix.de, mingo@redhat.com, dave.hansen@intel.com,
        luto@kernel.org, peterz@infradead.org,
        sathyanarayanan.kuppuswamy@linux.intel.com, aarcange@redhat.com,
        ak@linux.intel.com, dan.j.williams@intel.com, david@redhat.com,
        hpa@zytor.com, jgross@suse.com, jmattson@google.com,
        joro@8bytes.org, jpoimboe@redhat.com, knsathya@kernel.org,
        pbonzini@redhat.com, sdeep@vmware.com, seanjc@google.com,
        tony.luck@intel.com, vkuznets@redhat.com, wanpengli@tencent.com,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        Sean Christopherson <sean.j.christopherson@intel.com>
Subject: Re: [PATCH 04/26] x86/traps: Add #VE support for TDX guest
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On Wed, Dec 29, 2021 at 02:31:12AM +0300, Kirill A. Shutemov wrote:
> On Thu, Dec 23, 2021 at 08:45:40PM +0100, Borislav Petkov wrote:
> > What happens if the NMI handler triggers a #VE after all? Or where is it
> > enforced that TDX guests should set panic_on_oops?
> 
> Kernel will handle the #VE normally inside NMI handler. (We tested it once
> again, just in case.)
> 
> The critical part is that #VE must not be triggered in NMI entry code,
> before kernel is ready to handle nested NMIs.

Well, I can't read that in the commit message, maybe it needs expanding
on that aspect?

What I read is:

"Interrupts, including NMIs, are blocked by the hardware starting with
#VE delivery until TDGETVEINFO is called."

but this simply means that *if* you get a #VE anywhere, NMIs are masked
until TDGETVEINFO.

If you get a #VE during the NMI entry code, then you're toast...

> #VE cannot possibly happen there: no #VE-inducing instructions, code and
> data are in guest private memory.

Right, that. So we cannot get a #VE there.

> VMM can remove private memory from under us, but access to unaccepted (or
> missing) private memory leads to VM termination, not to #VE.

And that can't trigger a #VE either.

So I'm confused...

It sounds like you wanna say: no #VEs should happen during the NMI entry
code because of <raisins> and in order to prevent those, we don't use
insns causing #VE, etc. And private pages removed by the VM will simply
terminate the guest.

So what's up?

> tdx_virt_exception_user()/tdx_virt_exception_kernel() will be populated by
> following patches. The patch adds generic infrastructure for #VE handling.

Yeah, you either need to state that somewhere or keep changing those
functions as they evolve in the patchset. As it is, it just confuses
reviewers.

Thx.

-- 
Regards/Gruss,
    Boris.

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