linux-kernel - Re: [RFC v2-fix-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX guest

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Message-ID: <fe11bf3c-c0f8-7c25-8fc0-99bee9c1d164@linux.intel.com>
Date:   Tue, 25 May 2021 18:09:21 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.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-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX
 guest


On 5/24/2021 8:40 PM, Dan Williams wrote:
> On Mon, May 24, 2021 at 8:27 PM Andi Kleen <ak@...ux.intel.com> wrote:
>>
>> On 5/24/2021 7:49 PM, Dan Williams wrote:
>>> On Mon, May 24, 2021 at 7:13 PM Andi Kleen <ak@...ux.intel.com> wrote:
>>> [..]
>>>>> ...to explicitly error out a wbinvd use case before data is altered
>>>>> and wbinvd is needed.
>>>> I don't see any point of all of this. We really just want to be the same
>>>> as KVM. Not get into the business of patching a bazillion sub systems
>>>> that cannot be used in TDX anyways.
>>> Please let's not start this patch off with dubious claims of safety
>>> afforded by IgnorePAT. Instead make the true argument that wbinvd is
>>> known to be problematic in guests
>> That's just another reason to not support WBINVD, but I don't think it's
>> the main reason. The main reason is that it is simply not needed, unless
>> you do DMA in some form.
>>
>> (and yes I consider direct mapping of persistent memory with a complex
>> setup procedure a form of DMA -- my guess is that the reason that it
>> works in KVM is that it somehow activates the DMA code paths in KVM)
> No, it doesn't. Simply no one has tried to pass through the security
> interface of bare metal nvdimm to a guest, or enabled the security
> commands in a virtualized nvdimm.

Maybe a better term would be "external side effects". If you have 
something in IO domain which can notice a difference.

> If a guest supports a memory map it supports PMEM I struggle to see DMA anywhere in that equation.

Okay if that's happen to a TDX guest we have to start emulate WBINVD. 
But right now we don't need it.

I guess we can add a comment that says

"if someone wants to implement NVDIMM secure delete they would also need 
to implement this new hypercall"

>
>> IMNSHO that's the true reason.
> I do see why it would be attractive if IgnorePAT was a solid signal to
> ditch wbinvd support. However, it simply isn't, and to date nothing
> has cared trip over that gap.


I think we're getting into angels on a pinhead here.

The key point is that current TDX does not need WBINVD. I believe we 
agree on that.


-Andi