linux-kernel - Re: [PATCH Part2 RFC v4 10/40] x86/fault: Add support to handle the RMP fault for user address

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Message-ID: <aef6be8a-c93a-1aaa-57fe-116e70483542@amd.com>
Date:   Mon, 12 Jul 2021 11:24:16 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Dave Hansen <dave.hansen@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org
Cc:     brijesh.singh@....com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        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>, tony.luck@...el.com,
        npmccallum@...hat.com, brijesh.ksingh@...il.com
Subject: Re: [PATCH Part2 RFC v4 10/40] x86/fault: Add support to handle the
 RMP fault for user address



On 7/12/21 11:15 AM, Dave Hansen wrote:
> On 7/12/21 9:11 AM, Brijesh Singh wrote:
>>> Please fix this code to handle hugetlbfs along with any other non-THP
>>> source of level>0 mappings.  DAX comes to mind.  "Handle" can mean
>>> rejecting these.  You don't have to find some way to split them and make
>>> the VM work, just fail safely, ideally as early as possible.
>>>
>>> To me, this is a fundamental requirement before this code can be
>>> accepted.
>>
>> Understood, if userspace decided to use the hugetlbfs backing pages then
>> I believe earliest we can detect is when we go about adding the pages in
>> the RMP table. I'll add a check, and fail the page state change.
> 
> Really?  You had to feed the RMP entries from *some* mapping in the
> first place.  Is there a reason the originating mapping can't be checked
> at that point instead of waiting for the fault?
> 

Apologies if I was not clear in the messaging, that's exactly what I 
mean that we don't feed RMP entries during the page state change.

The sequence of the operation is:

1. Guest issues a VMGEXIT (page state change) to add a page in the RMP
2. Hyperivosr adds the page in the RMP table.

The check will be inside the hypervisor (#2), to query the backing page 
type, if the backing page is from the hugetlbfs, then don't add the page 
in the RMP, and fail the page state change VMGEXIT.

-Brijesh