linux-kernel - Re: [PATCH V1 3/6] xen/virtio: Add option to restrict memory access under Xen

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Date:   Tue, 26 Apr 2022 11:36:40 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Oleksandr <olekstysh@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Julien Grall <julien@....org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH V1 3/6] xen/virtio: Add option to restrict memory access
 under Xen

On 26.04.22 10:41, Borislav Petkov wrote:
> On Tue, Apr 26, 2022 at 07:16:16AM +0200, Juergen Gross wrote:
>> Christoph suggested (rather firmly) this would be the way to go.
> 
> Yeah, I saw it but I don't think it is the right way to go.
> 
> What happens the next time a guest needs to query the platform
> underneath? Misuse these interfaces again?
> 
> Because people will see the Xen use and say, hey, look, I will use this
> for my funky HV too.
> 
> Even worse: what happens if Xen decides to implement SEV/TDX? Then
> you're in for a world of fun.

As the suggestion was to add another flag this wouldn't be a problem IMO.

But I agree that coco might be not the best way to go (as I wrote already).

> 
> Now, if we want to *extend* the interfaces to have something as generic
> as, say, platform_has() and that should be the way for generic kernel
> code running in the guest to query the platform capabilities, then sure,
> by all means.

I agree.

> 
>> This is needed on guest side at a rather hypervisor independent place.
>>
>> So a capability of some sort seems appropriate.
>>
>> Another suggestion of mine was to have a callback (or flag) in
>> struct x86_hyper_runtime for that purpose.
> 
> This becomes an issue if the HV is not x86 - then you need a different
> method of querying it, which then underneath will call the arch-specific
> interface.
> 
> I don't know how much of querying guests need to do and how they've been
> doing that so far. Depending on the requirements, we probably should
> think about a clean design from the get-go instead of homegrown things.

Yes.

platform_has() doesn't seem too bad IMO.

I will write a patch for starting the discussion.


Juergen

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