linux-kernel - Re: [PATCH Part2 v5 08/45] x86/fault: Add support to handle the RMP fault for user address

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Date:   Tue, 24 Aug 2021 18:42:43 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Brijesh Singh <brijesh.singh@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part2 v5 08/45] x86/fault: Add support to handle the RMP
 fault for user address

On Mon, Aug 23, 2021 at 07:50:22AM -0700, Dave Hansen wrote:
> It *has* to be done in KVM, IMNHO.
> 
> The core kernel really doesn't know much about SEV.  It *really* doesn't
> know when its memory is being exposed to a virtualization architecture
> that doesn't know how to split TLBs like every single one before it.
> 
> This essentially *must* be done at the time that the KVM code realizes
> that it's being asked to shove a non-splittable page mapping into the
> SEV hardware structures.
> 
> The only other alternative is raising a signal from the fault handler
> when the page can't be split.  That's a *LOT* nastier because it's so
> much later in the process.
> 
> It's either that, or figure out a way to split hugetlbfs (and DAX)
> mappings in a failsafe way.

Yes, I agree with that. KVM needs a check to disallow HugeTLB pages in
SEV-SNP guests, at least as a temporary workaround. When HugeTLBfs
mappings can be split into smaller pages the check can be removed.

Regards,

	Joerg