From: David Hildenbrand <david@redhat.com>
To: Sean Christopherson <seanjc@google.com>,
"Kirill A. Shutemov" <kirill@shutemov.name>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
Andy Lutomirski <luto@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Jim Mattson <jmattson@google.com>,
David Rientjes <rientjes@google.com>,
"Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
"Kleen, Andi" <andi.kleen@intel.com>,
"Yamahata, Isaku" <isaku.yamahata@intel.com>,
Erdem Aktas <erdemaktas@google.com>,
Steve Rutherford <srutherford@google.com>,
Peter Gonda <pgonda@google.com>,
x86@kernel.org, kvm@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [RFCv2 13/13] KVM: unmap guest memory using poisoned pages
Date: Mon, 19 Apr 2021 20:12:06 +0200 [thread overview]
Message-ID: <c1d8367c-d975-dd51-0e2f-c48a97fb62d9@redhat.com> (raw)
In-Reply-To: <YH3HWeOXFiCTZN4y@google.com>
On 19.04.21 20:09, Sean Christopherson wrote:
> On Mon, Apr 19, 2021, Kirill A. Shutemov wrote:
>> On Mon, Apr 19, 2021 at 04:01:46PM +0000, Sean Christopherson wrote:
>>> But fundamentally the private pages, are well, private. They can't be shared
>>> across processes, so I think we could (should?) require the VMA to always be
>>> MAP_PRIVATE. Does that buy us enough to rely on the VMA alone? I.e. is that
>>> enough to prevent userspace and unaware kernel code from acquiring a reference
>>> to the underlying page?
>>
>> Shared pages should be fine too (you folks wanted tmpfs support).
>
> Is that a conflict though? If the private->shared conversion request is kicked
> out to userspace, then userspace can re-mmap() the files as MAP_SHARED, no?
>
> Allowing MAP_SHARED for guest private memory feels wrong. The data can't be
> shared, and dirty data can't be written back to the file.
>
>> The poisoned pages must be useless outside of the process with the blessed
>> struct kvm. See kvm_pfn_map in the patch.
>
> The big requirement for kernel TDX support is that the pages are useless in the
> host. Regarding the guest, for TDX, the TDX Module guarantees that at most a
> single KVM guest can have access to a page at any given time. I believe the RMP
> provides the same guarantees for SEV-SNP.
>
> SEV/SEV-ES could still end up with corruption if multiple guests map the same
> private page, but that's obviously not the end of the world since it's the status
> quo today. Living with that shortcoming might be a worthy tradeoff if punting
> mutual exclusion between guests to firmware/hardware allows us to simplify the
> kernel implementation.
>
>>>> - Add a new GUP flag to retrive such pages from the userspace mapping.
>>>> Used only for private mapping population.
>>>
>>>> - Shared gfn ranges managed by userspace, based on hypercalls from the
>>>> guest.
>>>>
>>>> - Shared mappings get populated via normal VMA. Any poisoned pages here
>>>> would lead to SIGBUS.
>>>>
>>>> So far it looks pretty straight-forward.
>>>>
>>>> The only thing that I don't understand is at way point the page gets tied
>>>> to the KVM instance. Currently we do it just before populating shadow
>>>> entries, but it would not work with the new scheme: as we poison pages
>>>> on fault it they may never get inserted into shadow entries. That's not
>>>> good as we rely on the info to unpoison page on free.
>>>
>>> Can you elaborate on what you mean by "unpoison"? If the page is never actually
>>> mapped into the guest, then its poisoned status is nothing more than a software
>>> flag, i.e. nothing extra needs to be done on free.
>>
>> Normally, poisoned flag preserved for freed pages as it usually indicate
>> hardware issue. In this case we need return page to the normal circulation.
>> So we need a way to differentiate two kinds of page poison. Current patch
>> does this by adding page's pfn to kvm_pfn_map. But this will not work if
>> we uncouple poisoning and adding to shadow PTE.
>
> Why use PG_hwpoison then?
>
I already raised that reusing PG_hwpoison is not what we want. And I
repeat, to me this all looks like a big hack; some things you (Sena)
propose sound cleaner, at least to me.
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2021-04-19 18:12 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-16 15:40 [RFCv2 00/13] TDX and guest memory unmapping Kirill A. Shutemov
2021-04-16 15:40 ` [RFCv2 01/13] x86/mm: Move force_dma_unencrypted() to common code Kirill A. Shutemov
2021-04-16 15:40 ` [RFCv2 02/13] x86/kvm: Introduce KVM memory protection feature Kirill A. Shutemov
2021-04-16 16:10 ` Borislav Petkov
2021-04-19 10:10 ` Kirill A. Shutemov
2021-04-16 15:40 ` [RFCv2 03/13] x86/kvm: Make DMA pages shared Kirill A. Shutemov
2021-04-16 15:40 ` [RFCv2 04/13] x86/kvm: Use bounce buffers for KVM memory protection Kirill A. Shutemov
2021-04-16 16:21 ` Dave Hansen
2021-04-16 15:40 ` [RFCv2 05/13] x86/kvmclock: Share hvclock memory with the host Kirill A. Shutemov
2021-04-16 15:40 ` [RFCv2 06/13] x86/realmode: Share trampoline area if KVM memory protection enabled Kirill A. Shutemov
2021-04-19 16:49 ` Dave Hansen
2021-04-16 15:41 ` [RFCv2 07/13] mm: Add hwpoison_entry_to_pfn() and hwpoison_entry_to_page() Kirill A. Shutemov
2021-04-16 15:41 ` [RFCv2 08/13] mm/gup: Add FOLL_ALLOW_POISONED Kirill A. Shutemov
2021-04-16 15:41 ` [RFCv2 09/13] shmem: Fail shmem_getpage_gfp() on poisoned pages Kirill A. Shutemov
2021-04-16 15:41 ` [RFCv2 10/13] mm: Keep page reference for hwpoison entries Kirill A. Shutemov
2021-04-16 15:41 ` [RFCv2 11/13] mm: Replace hwpoison entry with present PTE if page got unpoisoned Kirill A. Shutemov
2021-04-16 15:41 ` [RFCv2 12/13] KVM: passdown struct kvm to hva_to_pfn_slow() Kirill A. Shutemov
2021-04-16 15:41 ` [RFCv2 13/13] KVM: unmap guest memory using poisoned pages Kirill A. Shutemov
2021-04-16 17:30 ` Sean Christopherson
2021-04-19 11:32 ` Xiaoyao Li
2021-04-19 14:26 ` Kirill A. Shutemov
2021-04-19 16:01 ` Sean Christopherson
2021-04-19 16:40 ` Kirill A. Shutemov
2021-04-19 18:09 ` Sean Christopherson
2021-04-19 18:12 ` David Hildenbrand [this message]
2021-04-19 18:53 ` Kirill A. Shutemov
2021-04-19 20:09 ` Sean Christopherson
2021-04-19 22:57 ` Kirill A. Shutemov
2021-04-20 17:13 ` Sean Christopherson
2021-05-21 12:31 ` Kirill A. Shutemov
2021-05-26 19:46 ` Sean Christopherson
2021-05-31 20:07 ` Kirill A. Shutemov
2021-06-02 17:51 ` Sean Christopherson
2021-06-02 23:33 ` Kirill A. Shutemov
2021-06-03 19:46 ` Sean Christopherson
2021-06-04 14:29 ` Kirill A. Shutemov
2021-06-04 17:16 ` Andy Lutomirski
2021-06-04 17:54 ` Kirill A. Shutemov
2021-04-16 16:46 ` [RFCv2 00/13] TDX and guest memory unmapping Matthew Wilcox
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