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

From: Borislav Petkov <bp@alien8.de>
To: Juergen Gross <jgross@suse.com>
Cc: Oleksandr <olekstysh@gmail.com>,
	Christoph Hellwig <hch@infradead.org>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	xen-devel@lists.xenproject.org, x86@kernel.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Julien Grall <julien@xen.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: [PATCH V1 3/6] xen/virtio: Add option to restrict memory access under Xen
Date: Tue, 26 Apr 2022 10:41:09 +0200	[thread overview]
Message-ID: <YmewJaxWS1KGVkTf@zn.tnic> (raw)
In-Reply-To: <7d89848a-3a1c-415d-957a-564ffdd3712d@suse.com>

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.

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.

> 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.

Thx.

-- 
Regards/Gruss,
    Boris.

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