linux-kernel - Re: [RFC v2-fix-v5 1/1] x86: Skip WBINVD instruction for VM guest

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Message-ID: <65ca2294-d36a-1a65-3bed-90f4fd7dd14a@intel.com>
Date:   Wed, 9 Jun 2021 16:55:39 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2-fix-v5 1/1] x86: Skip WBINVD instruction for VM guest

On 6/9/21 2:42 PM, Kuppuswamy, Sathyanarayanan wrote:
> On 6/9/21 2:38 PM, Dan Williams wrote:
>>> In TDX guests, these WBINVD operations cause #VE exceptions.  For debug,
>>> it would be ideal for the #VE handler to be able to WARN() when an
>>> unexpected WBINVD occurs. (<--- problem #2)
>> ...but it doesn't WARN() it triggers unhandled #VE, unless I missed
>> another patch that precedes this that turns it into a WARN()? If a
>> code path expects WBINVD for correct operation and the guest can't
>> execute that sounds fatal, not a WARN to me.
> 
> Yes. It is not WARN. It is a fatal unhandled exception.

That makes the problem statement a wee bit different, but it should
still be pretty easy to explain:

	In TDX guests, these WBINVD operations cause #VE exceptions.
	While some #VE exceptions can be handled, there is no recourse
	for a TDX guest to handle a WBINVD and it will panic(). (<---
	problem #2)