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Date: Fri, 1 Mar 2024 09:26:18 +0000
From: Wei Liu <wei.liu@kernel.org>
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Subject: Re: [PATCH v4 0/3] x86/hyperv: Mark CoCo VM pages not present when
 changing encrypted state
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On Mon, Jan 15, 2024 at 06:20:05PM -0800, mhkelley58@gmail.com wrote:
> From: Michael Kelley <mhklinux@outlook.com>
> 
> In a CoCo VM, when transitioning memory from encrypted to decrypted, or
> vice versa, the caller of set_memory_encrypted() or set_memory_decrypted()
> is responsible for ensuring the memory isn't in use and isn't referenced
> while the transition is in progress.  The transition has multiple steps,
> and the memory is in an inconsistent state until all steps are complete.
> A reference while the state is inconsistent could result in an exception
> that can't be cleanly fixed up.
> 
> However, the kernel load_unaligned_zeropad() mechanism could cause a stray
> reference that can't be prevented by the caller of set_memory_encrypted()
> or set_memory_decrypted(), so there's specific code to handle this case.
> But a CoCo VM running on Hyper-V may be configured to run with a paravisor,
> with the #VC or #VE exception routed to the paravisor. There's no
> architectural way to forward the exceptions back to the guest kernel, and
> in such a case, the load_unaligned_zeropad() specific code doesn't work.
> 
> To avoid this problem, mark pages as "not present" while a transition
> is in progress. If load_unaligned_zeropad() causes a stray reference, a
> normal page fault is generated instead of #VC or #VE, and the
> page-fault-based fixup handlers for load_unaligned_zeropad() resolve the
> reference. When the encrypted/decrypted transition is complete, mark the
> pages as "present" again.
> 
> This version of the patch series marks transitioning pages "not present"
> only when running as a Hyper-V guest with a paravisor. Previous
> versions[1] marked transitioning pages "not present" regardless of the
> hypervisor and regardless of whether a paravisor is in use.  That more
> general use had the benefit of decoupling the load_unaligned_zeropad()
> fixup from CoCo VM #VE and #VC exception handling.  But the implementation
> was problematic for SEV-SNP because the SEV-SNP hypervisor callbacks
> require a valid virtual address, not a physical address like with TDX and
> the Hyper-V paravisor.  Marking the transitioning pages "not present"
> causes the virtual address to not be valid, and the PVALIDATE
> instruction in the SEV-SNP callback fails. Constructing a temporary
> virtual address for this purpose is slower and adds complexity that
> negates the benefits of the more general use. So this version narrows
> the applicability of the approach to just where it is required
> because of the #VC and #VE exceptions being routed to a paravisor.
> 
> The previous version minimized the TLB flushing done during page
> transitions between encrypted and decrypted. Because this version
> marks the pages "not present" in hypervisor specific callbacks and
> not in __set_memory_enc_pgtable(), doing such optimization is more
> difficult to coordinate. But the page transitions are not a hot path,
> so this version eschews optimization of TLB flushing in favor of
> simplicity.
> 
> Since this version no longer touches __set_memory_enc_pgtable(),
> I've also removed patches that add comments about error handling
> in that function.  Rick Edgecombe has proposed patches to improve
> that error handling, and I'll leave those comments to Rick's
> patches.
> 
> Patch 1 handles implications of the hypervisor callbacks needing
> to do virt-to-phys translations on pages that are temporarily
> marked not present.
> 
> Patch 2 makes the existing set_memory_p() function available for
> use in the hypervisor callbacks.
> 
> Patch 3 is the core change that marks the transitioning pages
> as not present.
> 
> This patch set is based on the linux-next20240103 code tree.
> 
> Changes in v4:
> * Patch 1: Updated comment in slow_virt_to_phys() to reduce the
>   likelihood of the comment becoming stale.  The new comment
>   describes the requirement to work with leaf PTE not present,
>   but doesn't directly reference the CoCo hypervisor callbacks.
>   [Rick Edgecombe]
> * Patch 1: Decomposed a complex line-wrapped statement into
>   multiple statements for ease of understanding. No functional
>   change compared with v3. [Kirill Shutemov]
> * Patch 3: Fixed handling of memory allocation errors. [Rick
>   Edgecombe]
> 
> Changes in v3:
> * Major rework and simplification per discussion above.
> 
> Changes in v2:
> * Added Patches 3 and 4 to deal with the failure on SEV-SNP
>   [Tom Lendacky]
> * Split the main change into two separate patches (Patch 5 and
>   Patch 6) to improve reviewability and to offer the option of
>   retaining both hypervisor callbacks.
> * Patch 5 moves set_memory_p() out of an #ifdef CONFIG_X86_64
>   so that the code builds correctly for 32-bit, even though it
>   is never executed for 32-bit [reported by kernel test robot]
> 
> [1] https://lore.kernel.org/lkml/20231121212016.1154303-1-mhklinux@outlook.com/
> 
> Michael Kelley (3):
>   x86/hyperv: Use slow_virt_to_phys() in page transition hypervisor
>     callback
>   x86/mm: Regularize set_memory_p() parameters and make non-static
>   x86/hyperv: Make encrypted/decrypted changes safe for
>     load_unaligned_zeropad()

Applied. Thanks.

The changes to mm code are mostly cosmetic. The changes had been pending
for a while without any objections from the maintainers, so I picked
them up as well.

If this becomes problematic, please let me know.

> 
>  arch/x86/hyperv/ivm.c             | 65 ++++++++++++++++++++++++++++---
>  arch/x86/include/asm/set_memory.h |  1 +
>  arch/x86/mm/pat/set_memory.c      | 24 +++++++-----
>  3 files changed, 75 insertions(+), 15 deletions(-)
> 
> -- 
> 2.25.1
>