Re: [PATCH v2 0/3] RFC: Solve several problems in stage 2 translation

[wangyanan (Y)]

Hi Will, Marc,

Gently Ping.

Is there any comments about this V2 series?


Many thanks,

Yanan.

On 2020/12/16 20:28, Yanan Wang wrote:
> Hi, this is the second version, thanks for reading.
>
> PATCH1/3:
> Procedures of hyp stage 1 mapping and guest stage 2 mapping are different, but
> they are tied closely by function kvm_set_valid_leaf_pte(). So separate them by
> rewriting kvm_set_valid_leaf_pte().
>
> PATCH2/3:
> To avoid unnecessary update and small loops, add prejudgement in the translation
> fault handler: Skip updating the PTE with break-before-make if we are trying to
> recreate the exact same mapping or only change the access permissions. Actually,
> change of permissions will be handled through the relax_perms path next time if
> necessary.
>
> (1) If there are some vCPUs accessing the same GPA at the same time and the leaf
> PTE is not set yet, then they will all cause translation faults and the first vCPU
> holding mmu_lock will set valid leaf PTE, and the others will later update the old
> PTE with a new one if they are different.
>
> (2) When changing a leaf entry or a table entry with break-before-make, if there
> are some vCPUs accessing the same GPA just catch the moment when the target PTE
> is set invalid in a BBM procedure coincidentally, they will all cause translation
> faults and will later update the old PTE with a new one if they are different.
>
> The worst case can be like this: vCPU A causes a translation fault with RW prot and
> sets the leaf PTE with RW permissions, and then the next vCPU B with RO prot updates
> the PTE back to RO permissions with break-before-make. And the BBM-invalid moment
> may trigger more unnecessary translation faults, then some useless small loops might
> occur which could lead to vCPU stuck.
>
> PATCH3/3:
> We now mark the page dirty and set the bitmap before calling fault handlers in
> user_mem_abort(), and we might end up having spurious dirty pages if update of
> permissions or mapping has failed. So, mark the page dirty only if the fault is
> handled successfully.
>
> Let the guest directly enter again but not return to userspace if we were trying
> to recreate the same mapping or only change access permissions with BBM, which is
> not permitted in the mapping path.
>
> Changes from v1:
> - Make part of the diff as an independent patch (PATCH1/3),
>    and add Will's Signed-off-by.
> - Use *return -EPERM* way when changing permissions only in the mapping path.
> - Add a new patch (PATCH3/3).
>
> Yanan Wang (3):
>    KVM: arm64: Decouple partial code of hyp stage 1 mapping and guest
>      stage 2 mapping
>    KVM: arm64: Add prejudgement for relaxing permissions only case in
>      stage2 translation fault handler
>    KVM: arm64: Mark the page dirty only if the fault is handled
>      successfully
>
>   arch/arm64/kvm/hyp/pgtable.c | 78 ++++++++++++++++++++----------------
>   arch/arm64/kvm/mmu.c         | 18 +++++++--
>   2 files changed, 58 insertions(+), 38 deletions(-)