linux-kernel - Re: [RFCv2 15/16] KVM: Unmap protected pages from direct mapping

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Message-ID: <2759b4bf-e1e3-d006-7d86-78a40348269d@redhat.com>
Date:   Tue, 20 Oct 2020 15:20:13 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Mike Rapoport <rppt@...ux.ibm.com>
Cc:     David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Will Drewry <wad@...omium.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Kleen, Andi" <andi.kleen@...el.com>,
        Liran Alon <liran.alon@...cle.com>,
        Mike Rapoport <rppt@...nel.org>, x86@...nel.org,
        kvm@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFCv2 15/16] KVM: Unmap protected pages from direct mapping

On 20.10.20 14:18, David Hildenbrand wrote:
> On 20.10.20 08:18, Kirill A. Shutemov wrote:
>> If the protected memory feature enabled, unmap guest memory from
>> kernel's direct mappings.
> 
> Gah, ugly. I guess this also defeats compaction, swapping, ... oh gosh.
> As if all of the encrypted VM implementations didn't bring us enough
> ugliness already (SEV extensions also don't support reboots, but can at
> least kexec() IIRC).
> 
> Something similar is done with secretmem [1]. And people don't seem to
> like fragmenting the direct mapping (including me).
> 
> [1] https://lkml.kernel.org/r/20200924132904.1391-1-rppt@kernel.org
> 

I just thought "hey, we might have to replace pud/pmd mappings by page
tables when calling kernel_map_pages", this can fail with -ENOMEM, why
isn't there proper error handling.

Then I dived into __kernel_map_pages() which states:

"The return value is ignored as the calls cannot fail. Large pages for
identity mappings are not used at boot time and hence no memory
allocations during large page split."

I am probably missing something important, but how is calling
kernel_map_pages() safe *after* booting?! I know we use it for
debug_pagealloc(), but using it in a production-ready feature feels
completely irresponsible. What am I missing?


-- 
Thanks,

David / dhildenb