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

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Date:   Mon, 19 Apr 2021 11:33:08 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Brijesh Singh <brijesh.singh@....com>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org, kvm@...r.kernel.org, linux-crypto@...r.kernel.org,
        ak@...ux.intel.com, herbert@...dor.apana.org.au,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        "H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [RFC Part2 PATCH 04/30] x86/mm: split the physmap when adding the
 page in RMP table

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.