Re: [RFC Part2 PATCH 04/30] x86/mm: split the physmap when adding the page in RMP table

From: Dave Hansen <dave.hansen@intel.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Brijesh Singh <brijesh.singh@amd.com>,
	Borislav Petkov <bp@alien8.de>,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	kvm@vger.kernel.org, linux-crypto@vger.kernel.org,
	ak@linux.intel.com, herbert@gondor.apana.org.au,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
	"H. Peter Anvin" <hpa@zytor.com>, Tony Luck <tony.luck@intel.com>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	David Rientjes <rientjes@google.com>,
	Sean Christopherson <seanjc@google.com>,
	Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [RFC Part2 PATCH 04/30] x86/mm: split the physmap when adding the page in RMP table
Date: Mon, 19 Apr 2021 11:33:08 -0700	[thread overview]
Message-ID: <535400b4-0593-a7ca-1548-532ee1fefbd7@intel.com> (raw)
In-Reply-To: <B17112AE-8848-48B0-997D-E1A3D79BD395@amacapital.net>

On 4/19/21 11:10 AM, Andy Lutomirski wrote:
> I’m confused by this scenario. This should only affect physical pages
> that are in the 2M area that contains guest memory. But, if we have a
> 2M direct map PMD entry that contains kernel data and guest private
> memory, we’re already in a situation in which the kernel touching
> that memory would machine check, right?

Not machine check, but page fault.  Do machine checks even play a
special role in SEV-SNP?  I thought that was only TDX?

My point was just that you can't _easily_ do the 2M->4k kernel mapping
demotion in a page fault handler, like I think Borislav was suggesting.

> ISTM we should fully unmap any guest private page from the kernel and
> all host user pagetables before actually making it be a guest private
> page.

Yes, that sounds attractive.  Then, we'd actually know if the host
kernel was doing stray reads somehow because we'd get a fault there too.