linux-kernel - Re: [PATCH 0/7] KVM: TDX SEPT SEAMCALL retry

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Message-ID: <cd099216-5fc7-4a79-8d35-b87c356e122b@redhat.com>
Date: Wed, 15 Jan 2025 17:43:51 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Yan Zhao <yan.y.zhao@...el.com>, seanjc@...gle.com, kvm@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, rick.p.edgecombe@...el.com,
 kai.huang@...el.com, adrian.hunter@...el.com, reinette.chatre@...el.com,
 xiaoyao.li@...el.com, tony.lindgren@...el.com, binbin.wu@...ux.intel.com,
 dmatlack@...gle.com, isaku.yamahata@...el.com, isaku.yamahata@...il.com
Subject: Re: [PATCH 0/7] KVM: TDX SEPT SEAMCALL retry

On 1/13/25 03:09, Yan Zhao wrote:
> This series aims to provide a clean solution to avoid the blind retries in
> the previous hack [1] in "TDX MMU Part 2," following the initial
> discussions to [2], further discussions in the RFC, and the PUCK [3].
> 
> A full analysis of the lock status for each SEAMCALL relevant to KVM is
> available at [4].
> 
> This series categorizes the SEPT-related SEAMCALLs (used for page
> installation and uninstallation) into three groups:
> 
> Group 1: tdh_mem_page_add().
>         - Invoked only during TD build time.
>         - Proposal: Return -EBUSY on TDX_OPERAND_BUSY.
>         - Patch 1.
> 
> Group 2: tdh_mem_sept_add(), tdh_mem_page_aug().
>         - Invoked for TD runtime page installation.
>         - Proposal: Retry locally in the TDX EPT violation handler for
>           RET_PF_RETRY.
>         - Patches 2-3.
> 
> Group 3: tdh_mem_range_block(), tdh_mem_track(), tdh_mem_page_remove().
>         - Invoked for page uninstallation, with KVM mmu_lock held for write.
>         - Proposal: Kick off vCPUs and no vCPU entry on TDX_OPERAND_BUSY.
>         - Patch 4.
> 
> Patches 5/6/7 are fixup patches:
> Patch 5: Return -EBUSY instead of -EAGAIN when tdh_mem_sept_add() is busy.
> Patch 6: Remove the retry loop for tdh_phymem_page_wbinvd_hkid().
> Patch 7: Warn on force_immediate_exit in tdx_vcpu_run().
> 
> Code base: kvm-coco-queue 2f30b837bf7b.
> Applies to the tail since the dependence on
> commit 8e801e55ba8f ("KVM: TDX: Handle EPT violation/misconfig exit"),
> 
> Thanks
> Yan
> 
> RFC --> v1:
> - Split patch 1 in RFC into patches 1,2,3,5, and add new fixup patches 6/7.
> - Add contention analysis of tdh_mem_page_add() in patch 1 log.
> - Provide justification in patch 2 log and add checks for RET_PF_CONTINUE.
> - Use "a per-VM flag wait_for_sept_zap + KVM_REQ_OUTSIDE_GUEST_MODE"
>    instead of a arch-specific request to prevent vCPUs from TD entry in patch 4
>    (Sean).
> 
> RFC: https://lore.kernel.org/all/20241121115139.26338-1-yan.y.zhao@intel.com
> [1] https://lore.kernel.org/all/20241112073909.22326-1-yan.y.zhao@intel.com
> [2] https://lore.kernel.org/kvm/20240904030751.117579-10-rick.p.edgecombe@intel.com/
> [3] https://drive.google.com/drive/folders/1k0qOarKuZXpzRsKDtVeC5Lpl9-amJ6AJ?resourcekey=0-l9uVpVEBC34Uar1ReaqisQ
> [4] https://lore.kernel.org/kvm/ZuP5eNXFCljzRgWo@yzhao56-desk.sh.intel.com

Thanks, I applied this to kvm-coco-queue and patch 2 to kvm/queue.  It 
is spread all over the branch to make the dependencies clearer, so 
here's some ideas on how to include these.

Patches 6 and 7 should be squashed into the respective bases, as they 
have essentially no functional change.

For the rest, patch 1 can be treated as a fixup too, and I have two 
proposals.

First possibility, separate series:
* patches 1+5 are merged into a single patch.
* patches 3+4 become two more patches in this separate series

Second possibility, squash everything:
* patches 1+5 are squashed into the respective bases
* patches 3+4 are included in the EPT violation series

On the PUCK call I said that I prefer the first, mostly to keep track of 
who needs to handle TDX_OPERAND_BUSY, but if it makes it easier for Yan 
then feel free to go for the second.

Paolo

> Yan Zhao (7):
>    KVM: TDX: Return -EBUSY when tdh_mem_page_add() encounters
>      TDX_OPERAND_BUSY
>    KVM: x86/mmu: Return RET_PF* instead of 1 in kvm_mmu_page_fault()
>    KVM: TDX: Retry locally in TDX EPT violation handler on RET_PF_RETRY
>    KVM: TDX: Kick off vCPUs when SEAMCALL is busy during TD page removal
>    fixup! KVM: TDX: Implement hooks to propagate changes of TDP MMU
>      mirror page table
>    fixup! KVM: TDX: Implement hooks to propagate changes of TDP MMU
>      mirror page table
>    fixup! KVM: TDX: Implement TDX vcpu enter/exit path
> 
>   arch/x86/kvm/mmu/mmu.c          |  10 ++-
>   arch/x86/kvm/mmu/mmu_internal.h |  12 ++-
>   arch/x86/kvm/vmx/tdx.c          | 135 ++++++++++++++++++++++++++------
>   arch/x86/kvm/vmx/tdx.h          |   7 ++
>   4 files changed, 134 insertions(+), 30 deletions(-)
>