linux-kernel - Re: [PATCH Part2 RFC v4 07/40] x86/sev: Split the physmap when adding the page in RMP table

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Message-ID: <6c2eefa9-a7f2-399b-f38f-bfae580cdca9@amd.com>
Date:   Fri, 30 Jul 2021 11:10:01 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     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, 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>,
        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>, tony.luck@...el.com,
        npmccallum@...hat.com, brijesh.ksingh@...il.com
Subject: Re: [PATCH Part2 RFC v4 07/40] x86/sev: Split the physmap when adding
 the page in RMP table


On 7/30/21 6:31 AM, Vlastimil Babka wrote:
> On 7/15/21 9:38 PM, Brijesh Singh wrote:
>>
>> On 7/15/21 1:39 PM, Sean Christopherson wrote:
>>> On Thu, Jul 15, 2021, Brijesh Singh wrote:
>>>> The memfd_secrets uses the set_direct_map_{invalid,default}_noflush() and it
>>>> is designed to remove/add the present bit in the direct map. We can't use
>>>> them, because in our case the page may get accessed by the KVM (e.g
>>>> kvm_guest_write, kvm_guest_map etc).
>>> But KVM should never access a guest private page, i.e. the direct map should
>>> always be restored to PRESENT before KVM attempts to access the page.
>>>
>> Yes, KVM should *never* access the guest private pages. So, we could potentially
>> enhance the RMPUPDATE() to check for the assigned and act accordingly.
> I think I'm not the first one suggesting it, but IMHO the best solution would be
> to leave direct map alone (except maybe in some debugging/development mode where
> you could do the unmapping to catch unexpected host accesses), and once there's
> a situation with host accessing a shared page of the guest, it would temporarily
> kmap() it outside of the direct map. Shouldn't these situations be rare enough,
> and already recognizable due to the need to set up the page sharing in the first
> place, that this approach would be feasible?

The host accessing a guest shared is not rare, some shared pages are
accessed on every VM-Entry and Exit. However, there are limited number
of such pages and we could track and temporary kmap() outside of the
direct map. The main challenge is when a host pages are added as
'private' in the RMP table.

e.g

page = alloc_page(GFP_KERNEL_ACCOUNT);

// firmware context page

rmp_make_firmware(page_to_pfn(page), PG_LEVEL_4K);

or

// VMSA page

rmp_make_private(page_to_pfn(page), 0, sev->asid, PG_LEVEL_4K);

The page is added as 4K in the RMP entry; If the pfn is part of the
large direct map then hardware will raise an RMP violation and to
resolve the fault we must split the direct map.

thanks

>
>> Are you thinking something along the line of this:
>>
>> int rmpupdate(struct page *page, struct rmpupdate *val)
>> {
>>     ...
>>     
>>     /*
>>      * If page is getting assigned in the RMP entry then unmap
>>      * it from the direct map before its added in the RMP table.
>>      */
>>     if (val.assigned)
>>         set_direct_map_invalid_noflush(page_to_virt(page), 1);
>>
>>     ...
>>
>>     /*
>>      * If the page is getting unassigned then restore the mapping
>>      * in the direct map after its removed from the RMP table.
>>      */
>>     if (!val.assigned)
>>         set_direct_map_default_noflush(page_to_virt(page), 1);
>>     
>>     ...
>> }
>>
>> thanks