| Message ID | 20240718223519.1673835-1-coltonlewis@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: arm64: Move data barrier to end of split walk | expand |
[+Ricardo, as he wrote the original split walker] On Thu, Jul 18, 2024 at 10:35:19PM +0000, Colton Lewis wrote: > Moving the data barrier from stage2_split_walker to after the walk is > finished in kvm_pgtable_stage2_split results in a roughly 70% > reduction in Clear Dirty Log Time in dirty_log_perf_test (modified to > use eager page splitting) when using huge pages. This gain holds > steady through a range of vcpus used (tested 1-64) and memory > used (tested 1-64GB). > > This is safe to do because nothing else is using the page tables while > they are still being mapped and this is how other page table walkers > already function. None of them have a data barrier in the walker > itself. > > Signed-off-by: Colton Lewis <coltonlewis@google.com> > --- > arch/arm64/kvm/hyp/pgtable.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > index 9e2bbee77491..9788af2ca8c0 100644 > --- a/arch/arm64/kvm/hyp/pgtable.c > +++ b/arch/arm64/kvm/hyp/pgtable.c > @@ -1547,7 +1547,6 @@ static int stage2_split_walker(const struct kvm_pgtable_visit_ctx *ctx, > */ > new = kvm_init_table_pte(childp, mm_ops); > stage2_make_pte(ctx, new); > - dsb(ishst); > return 0; > } > > @@ -1559,8 +1558,11 @@ int kvm_pgtable_stage2_split(struct kvm_pgtable *pgt, u64 addr, u64 size, > .flags = KVM_PGTABLE_WALK_LEAF, > .arg = mc, > }; > + int ret; > > - return kvm_pgtable_walk(pgt, addr, size, &walker); > + ret = kvm_pgtable_walk(pgt, addr, size, &walker); > + dsb(ishst); > + return ret; > } This looks ok to me, but it would be great if Ricardo could have a look as well. Will
On Thu, Jul 18, 2024 at 10:35:19PM +0000, Colton Lewis wrote: > Moving the data barrier from stage2_split_walker to after the walk is > finished in kvm_pgtable_stage2_split results in a roughly 70% > reduction in Clear Dirty Log Time in dirty_log_perf_test (modified to > use eager page splitting) when using huge pages. This gain holds > steady through a range of vcpus used (tested 1-64) and memory > used (tested 1-64GB). > > This is safe to do because nothing else is using the page tables while > they are still being mapped and this is how other page table walkers > already function. None of them have a data barrier in the walker > itself. nitpick: in the interest of the reader, it'd be a good idea to state explicitly what purpose the DSB serves, which is to guarantee page table updates have been made visible to hardware table walker. Relative ordering of table PTEs to table contents comes from the fact that stage2_make_pte() has release semantics.
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c index 9e2bbee77491..9788af2ca8c0 100644 --- a/arch/arm64/kvm/hyp/pgtable.c +++ b/arch/arm64/kvm/hyp/pgtable.c @@ -1547,7 +1547,6 @@ static int stage2_split_walker(const struct kvm_pgtable_visit_ctx *ctx, */ new = kvm_init_table_pte(childp, mm_ops); stage2_make_pte(ctx, new); - dsb(ishst); return 0; } @@ -1559,8 +1558,11 @@ int kvm_pgtable_stage2_split(struct kvm_pgtable *pgt, u64 addr, u64 size, .flags = KVM_PGTABLE_WALK_LEAF, .arg = mc, }; + int ret; - return kvm_pgtable_walk(pgt, addr, size, &walker); + ret = kvm_pgtable_walk(pgt, addr, size, &walker); + dsb(ishst); + return ret; } int __kvm_pgtable_stage2_init(struct kvm_pgtable *pgt, struct kvm_s2_mmu *mmu,
Moving the data barrier from stage2_split_walker to after the walk is finished in kvm_pgtable_stage2_split results in a roughly 70% reduction in Clear Dirty Log Time in dirty_log_perf_test (modified to use eager page splitting) when using huge pages. This gain holds steady through a range of vcpus used (tested 1-64) and memory used (tested 1-64GB). This is safe to do because nothing else is using the page tables while they are still being mapped and this is how other page table walkers already function. None of them have a data barrier in the walker itself. Signed-off-by: Colton Lewis <coltonlewis@google.com> --- arch/arm64/kvm/hyp/pgtable.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)