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SubjectRe: [PATCH 00/15] KVM: x86/mmu: TLB fixes and related cleanups
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On 10/06/21 01:42, Sean Christopherson wrote:
> Fixes for two (very) theoretical TLB flushing bugs (patches 1 and 4),
> and clean ups on top to (hopefully) consolidate and simplifiy the TLB
> flushing logic.
>
> The basic gist of the TLB flush and MMU sync code shuffling is to stop
> relying on the logic in __kvm_mmu_new_pgd() (but keep it for forced
> flushing), and instead handle the flush+sync logic in the caller
> independent from whether or not the "fast" switch occurs.
>
> I spent a fair bit of time trying to shove the necessary logic down into
> __kvm_mmu_new_pgd(), but it always ended up a complete mess because the
> requirements and contextual information is always different. The rules
> for MOV CR3 are different from nVMX transitions (and those vary based on
> EPT+VPID), and nSVM will be different still (once it adds proper TLB
> handling). In particular, I like that nVMX no longer has special code
> for synchronizing the MMU when using shadowing paging and instead relies
> on the common rules for TLB flushing.
>
> Note, this series (indirectly) relies heavily on commit b53e84eed08b
> ("KVM: x86: Unload MMU on guest TLB flush if TDP disabled to force MMU
> sync"), as it uses KVM_REQ_TLB_FLUSH_GUEST (was KVM_REQ_HV_TLB_FLUSH)
> to do the TLB flush _and_ the MMU sync in non-PV code.
>
> Tested all combinations for i386, EPT, NPT, and shadow paging. I think...
>
> The EPTP switching and INVPCID single-context changes in particular lack
> meaningful coverage in kvm-unit-tests+Linux. Long term it's on my todo
> list to remedy that, but realistically I doubt I'll get it done anytime
> soon.
>
> To test EPTP switching, I hacked L1 to set up a duplicate top-level EPT
> table, copy the "real" table to the duplicate table on EPT violation,
> populate VMFUNC.EPTP_LIST with the two EPTPs, expose VMFUNC.EPTP_SWITCH
> to L2. I then hacked L2 to do an EPTP switch to a random (valid) EPTP
> index on every task switch.
>
> To test INVPCID single-context I modified L1 to iterate over all possible
> PCIDs using INVPCID single-context in native_flush_tlb_global(). I also
> verified that the guest crashed if it didn't do any INVPCID at all
> (interestingly, the guest made it through boot without the flushes when
> EPT was enabled, which implies the missing MMU sync on INVPCID was the
> source of the crash, not a stale TLB entry).
>
> Sean Christopherson (15):
> KVM: nVMX: Sync all PGDs on nested transition with shadow paging
> KVM: nVMX: Ensure 64-bit shift when checking VMFUNC bitmap
> KVM: nVMX: Don't clobber nested MMU's A/D status on EPTP switch
> KVM: x86: Invalidate all PGDs for the current PCID on MOV CR3 w/ flush
> KVM: x86: Uncondtionally skip MMU sync/TLB flush in MOV CR3's PGD
> switch
> KVM: nSVM: Move TLB flushing logic (or lack thereof) to dedicated
> helper
> KVM: x86: Drop skip MMU sync and TLB flush params from "new PGD"
> helpers
> KVM: nVMX: Consolidate VM-Enter/VM-Exit TLB flush and MMU sync logic
> KVM: nVMX: Free only guest_mode (L2) roots on INVVPID w/o EPT
> KVM: x86: Use KVM_REQ_TLB_FLUSH_GUEST to handle INVPCID(ALL) emulation
> KVM: nVMX: Use fast PGD switch when emulating VMFUNC[EPTP_SWITCH]
> KVM: x86: Defer MMU sync on PCID invalidation
> KVM: x86: Drop pointless @reset_roots from kvm_init_mmu()
> KVM: nVMX: WARN if subtly-impossible VMFUNC conditions occur
> KVM: nVMX: Drop redundant checks on vmcs12 in EPTP switching emulation
>
> arch/x86/include/asm/kvm_host.h | 6 +-
> arch/x86/kvm/hyperv.c | 2 +-
> arch/x86/kvm/mmu.h | 2 +-
> arch/x86/kvm/mmu/mmu.c | 57 ++++++++-----
> arch/x86/kvm/svm/nested.c | 40 ++++++---
> arch/x86/kvm/vmx/nested.c | 139 ++++++++++++--------------------
> arch/x86/kvm/x86.c | 75 ++++++++++-------
> 7 files changed, 169 insertions(+), 152 deletions(-)
>

I tried this a couple times but was blocked on what is essentially your
first patch, so thanks! Patches queued for 5.14.

Paolo

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