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Subject: Re: [PATCH Part2 v5 08/45] x86/fault: Add support to handle the RMP
 fault for user address
To:     Brijesh Singh <brijesh.singh@amd.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-coco@lists.linux.dev, linux-mm@kvack.org,
        linux-crypto@vger.kernel.org
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Sergio Lopez <slp@redhat.com>, Peter Gonda <pgonda@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Dov Murik <dovmurik@linux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@ibm.com>,
        Borislav Petkov <bp@alien8.de>,
        Michael Roth <michael.roth@amd.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Andi Kleen <ak@linux.intel.com>, tony.luck@intel.com,
        marcorr@google.com, sathyanarayanan.kuppuswamy@linux.intel.com
References: <20210820155918.7518-1-brijesh.singh@amd.com>
 <20210820155918.7518-9-brijesh.singh@amd.com>
 <f6d778a0-840d-8c9c-392d-5c9ffcc0bdc6@intel.com>
 <19599ede-9fc5-25e1-dcb3-98aafd8b7e87@amd.com>
From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <3f426ef8-060e-ccc9-71b9-2448f2582a30@intel.com>
Date:   Mon, 23 Aug 2021 07:50:22 -0700
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In-Reply-To: <19599ede-9fc5-25e1-dcb3-98aafd8b7e87@amd.com>
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Precedence: bulk

On 8/23/21 7:36 AM, Brijesh Singh wrote:
> On 8/23/21 9:20 AM, Dave Hansen wrote:
>> On 8/20/21 8:58 AM, Brijesh Singh wrote:
>>> +static int handle_split_page_fault(struct vm_fault *vmf)
>>> +{
>>> +    if (!IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT))
>>> +        return VM_FAULT_SIGBUS;
>>> +
>>> +    __split_huge_pmd(vmf->vma, vmf->pmd, vmf->address, false, NULL);
>>> +    return 0;
>>> +}
>>
>> We had a whole conversation the last time this was posted about huge
>> page types *other* than THP.  I don't see any comprehension of those
>> types or what would happen if one of those was used with SEV-SNP.
>>
>> What was the result of those review comments?
> 
> Based on previous review comments Sean was not keen on KVM having
> perform this detection and abort the guest SEV-SNP VM launch. So, I
> didn't implemented the check and waiting for more discussion before
> going at it.

OK.  But, you need to *acknowledge* the situation somewhere.  Maybe the
cover letter of the series, maybe in this changelog.

As it stands, it looks like you're simply ignoring _some_ reviewer feedback.

> SEV-SNP guest requires the VMM to register the guest backing pages
> before the VM launch. Personally, I would prefer KVM to check the
> backing page type during the registration and fail to register if its
> hugetlbfs (and others) to avoid us get into situation where we could not
> split the hugepage.

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.