From: Fuad Tabba <tabba@google.com>
To: Oliver Upton <oliver.upton@linux.dev>
Cc: kvmarm@lists.linux.dev, maz@kernel.org, will@kernel.org,
qperret@google.com, seanjc@google.com, alexandru.elisei@arm.com,
catalin.marinas@arm.com, philmd@linaro.org, james.morse@arm.com,
suzuki.poulose@arm.com, mark.rutland@arm.com,
broonie@kernel.org, joey.gouly@arm.com, rananta@google.com,
smostafa@google.com
Subject: Re: [PATCH v2 09/47] KVM: arm64: Avoid BBM when changing only s/w bits in Stage-2 PTE
Date: Tue, 16 Apr 2024 19:06:37 +0100 [thread overview]
Message-ID: <CA+EHjTz6WZD7VYie1CTnv8F4CAzk+uVJA6hrt4oF0evhHFdwvQ@mail.gmail.com> (raw)
In-Reply-To: <Zh6ujO8qbVviKlEk@linux.dev>
Hi Oliver,
On Tue, Apr 16, 2024 at 6:00 PM Oliver Upton <oliver.upton@linux.dev> wrote:
>
> Hi Fuad,
>
> On Tue, Apr 16, 2024 at 10:56:00AM +0100, Fuad Tabba wrote:
> > From: Will Deacon <will@kernel.org>
> >
> > Break-before-make (BBM) can be expensive, as transitioning via an
> > invalid mapping (i.e. the "break" step) requires the completion of TLB
> > invalidation and can also cause other agents to fault concurrently on
> > the invalid mapping.
> >
> > Since BBM is not required when changing only the software bits of a PTE,
> > avoid the sequence in this case and just update the PTE directly.
> >
> > Signed-off-by: Will Deacon <will@kernel.org>
> > Signed-off-by: Fuad Tabba <tabba@google.com>
> > ---
> > arch/arm64/kvm/hyp/pgtable.c | 15 +++++++++++++++
> > 1 file changed, 15 insertions(+)
> >
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 67647b853c9b..30a9a503c477 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -979,6 +979,21 @@ static int stage2_map_walker_try_leaf(const struct kvm_pgtable_visit_ctx *ctx,
> > if (!stage2_pte_needs_update(ctx->old, new))
> > return -EAGAIN;
> >
> > + /* If we're only changing software bits, then store them and go! */
> > + if (!kvm_pgtable_walk_shared(ctx) &&
> > + !((ctx->old ^ new) & ~KVM_PTE_LEAF_ATTR_HI_SW)) {
> > + bool old_is_counted = stage2_pte_is_counted(ctx->old);
> > +
> > + if (old_is_counted != stage2_pte_is_counted(new)) {
> > + if (old_is_counted)
> > + mm_ops->put_page(ctx->ptep);
> > + else
> > + mm_ops->get_page(ctx->ptep);
> > + }
> > + WRITE_ONCE(*ctx->ptep, new);
>
> IIUC, you're using a naked WRITE_ONCE() to avoid a error path that
> cleans up the reference count. That's fine, but can you change this to a
> WARN_ON_ONCE(stage2_try_set_pte())? Should this ever be changed to
> become a shared walk w/o gracefully handling a cleanup here I'd hope the
> warning would catch someone's attention.
I think you mean WARN_ON_ONCE(!stage2_try_set_pte(...)). I can do
that, but for now, warnings in protected mode are fatal. Should I do
that anyway?
Cheers,
/fuad
> --
> Thanks,
> Oliver
next prev parent reply other threads:[~2024-04-16 18:07 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-16 9:55 [PATCH v2 00/47] KVM: arm64: Preamble for pKVM Fuad Tabba
2024-04-16 9:55 ` [PATCH v2 01/47] KVM: arm64: Allocate per-cpu memory for the host fpsimd state in pKVM Fuad Tabba
2024-04-16 10:57 ` Mark Brown
2024-04-16 12:54 ` Marc Zyngier
2024-04-16 13:08 ` Marc Zyngier
2024-04-16 13:10 ` Fuad Tabba
2024-04-16 9:55 ` [PATCH v2 02/47] KVM: arm64: Do not re-initialize the KVM lock Fuad Tabba
2024-04-16 9:55 ` [PATCH v2 03/47] KVM: arm64: Issue CMOs when tearing down guest s2 pages Fuad Tabba
2024-04-16 9:55 ` [PATCH v2 04/47] KVM: arm64: Avoid BUG-ing from the host abort path Fuad Tabba
2024-04-16 9:55 ` [PATCH v2 05/47] KVM: arm64: Fix comment for __pkvm_vcpu_init_traps() Fuad Tabba
2024-04-16 9:55 ` [PATCH v2 06/47] KVM: arm64: Change kvm_handle_mmio_return() return polarity Fuad Tabba
2024-04-16 9:55 ` [PATCH v2 07/47] KVM: arm64: Move setting the page as dirty out of the critical section Fuad Tabba
2024-04-16 9:55 ` [PATCH v2 08/47] KVM: arm64: Check for PTE validity when checking for executable/cacheable Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 09/47] KVM: arm64: Avoid BBM when changing only s/w bits in Stage-2 PTE Fuad Tabba
2024-04-16 16:59 ` Oliver Upton
2024-04-16 18:06 ` Fuad Tabba [this message]
2024-04-16 18:19 ` Oliver Upton
2024-04-16 9:56 ` [PATCH v2 10/47] KVM: arm64: Support TLB invalidation in guest context Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 11/47] KVM: arm64: Simplify vgic-v3 hypercalls Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 12/47] KVM: arm64: Add is_pkvm_initialized() helper Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 13/47] KVM: arm64: Introduce predicates to check for protected state Fuad Tabba
2024-04-16 17:18 ` Oliver Upton
2024-04-16 18:07 ` Fuad Tabba
2024-04-16 18:10 ` Oliver Upton
2024-04-16 18:12 ` Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 14/47] KVM: arm64: Split up nvhe/fixed_config.h Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 15/47] KVM: arm64: Move pstate reset value definitions to kvm_arm.h Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 16/47] KVM: arm64: Clarify rationale for ZCR_EL1 value restored on guest exit Fuad Tabba
2024-04-16 10:47 ` Mark Brown
2024-04-16 13:38 ` Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 17/47] KVM: arm64: Refactor calculating SVE state size to use helpers Fuad Tabba
2024-04-16 10:48 ` Mark Brown
2024-04-16 9:56 ` [PATCH v2 18/47] KVM: arm64: Do not map the host fpsimd state to hyp in pKVM Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 19/47] KVM: arm64: Move some kvm_psci functions to a shared header Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 20/47] KVM: arm64: Refactor reset_mpidr() to extract its computation Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 21/47] KVM: arm64: Refactor kvm_vcpu_enable_ptrauth() for hyp use Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 22/47] KVM: arm64: Refactor enter_exception64() Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 23/47] KVM: arm64: Add PC_UPDATE_REQ flags covering all PC updates Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 24/47] KVM: arm64: Add vcpu flag copy primitive Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 25/47] KVM: arm64: Introduce gfn_to_memslot_prot() Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 26/47] KVM: arm64: Do not use the hva in kvm_handle_guest_abort() Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 27/47] KVM: arm64: Introduce hyp_rwlock_t Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 28/47] KVM: arm64: Add atomics-based checking refcount implementation at EL2 Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 29/47] KVM: arm64: Use atomic refcount helpers for 'struct hyp_page::refcount' Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 30/47] KVM: arm64: Remove locking from EL2 allocation fast-paths Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 31/47] KVM: arm64: Reformat/beautify PTP hypercall documentation Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 32/47] KVM: arm64: Rename firmware pseudo-register documentation file Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 33/47] KVM: arm64: Document the KVM/arm64-specific calls in hypercalls.rst Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 34/47] KVM: arm64: Prevent kmemleak from accessing .hyp.data Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 35/47] KVM: arm64: Do not set the virtual timer offset for protected vCPUs Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 36/47] KVM: arm64: Check directly whether a vcpu is protected Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 37/47] KVM: arm64: Trap debug break and watch from guest Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 38/47] KVM: arm64: Restrict protected VM capabilities Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 39/47] KVM: arm64: Do not support MTE for protected VMs Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 40/47] KVM: arm64: Move pkvm_vcpu_init_traps() to hyp vcpu init Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 41/47] KVM: arm64: Fix initializing traps in protected mode Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 42/47] KVM: arm64: Advertise GICv3 sysreg interface to protected guests Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 43/47] KVM: arm64: Force injection of a data abort on NISV MMIO exit Fuad Tabba
2024-04-16 9:56 ` [PATCH v2 44/47] KVM: arm64: Remove unused vcpu parameter from guest_owns_fp_regs() Fuad Tabba
2024-04-16 10:49 ` Mark Brown
2024-04-16 9:56 ` [PATCH v2 45/47] KVM: arm64: Move guest_owns_fp_regs() to enable use in KVM code Fuad Tabba
2024-04-16 10:50 ` Mark Brown
2024-04-16 9:56 ` [PATCH v2 46/47] KVM: arm64: Add host_owns_fp_regs() Fuad Tabba
2024-04-16 10:51 ` Mark Brown
2024-04-16 9:56 ` [PATCH v2 47/47] KVM: arm64: Refactor checks for FP state ownership Fuad Tabba
2024-04-16 10:55 ` Mark Brown
2024-04-16 13:33 ` Marc Zyngier
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