[RFC PATCH 0/4] KVM: arm64: Improve efficiency of stage2 page table

[Alexandru Elisei]

Hi Yanan,

I wanted to review the patches, but unfortunately I get an error when trying to
apply the first patch in the series:

Applying: KVM: arm64: Move the clean of dcache to the map handler
error: patch failed: arch/arm64/kvm/hyp/pgtable.c:464
error: arch/arm64/kvm/hyp/pgtable.c: patch does not apply
error: patch failed: arch/arm64/kvm/mmu.c:882
error: arch/arm64/kvm/mmu.c: patch does not apply
Patch failed at 0001 KVM: arm64: Move the clean of dcache to the map handler
hint: Use 'git am --show-current-patch=diff' to see the failed patch
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

Tried this with Linux tags v5.11-rc1 to v5.11-rc7. It looks like pgtable.c and
mmu.c from your patch is different than what is found on upstream master. Did you
use another branch as the base for your patches?

Thanks,

Alex

On 2/8/21 11:22 AM, Yanan Wang wrote:
> Hi,
>
> This series makes some efficiency improvement of stage2 page table code,
> and there are some test results to present the performance changes, which
> were tested by a kvm selftest [1] that I have post:
> [1] https://lore.kernel.org/lkml/20210208090841.333724-1-wangyanan55@huawei.com/ 
>
> About patch 1:
> We currently uniformly clean dcache in user_mem_abort() before calling the
> fault handlers, if we take a translation fault and the pfn is cacheable.
> But if there are concurrent translation faults on the same page or block,
> clean of dcache for the first time is necessary while the others are not.
>
> By moving clean of dcache to the map handler, we can easily identify the
> conditions where CMOs are really needed and avoid the unnecessary ones.
> As it's a time consuming process to perform CMOs especially when flushing
> a block range, so this solution reduces much load of kvm and improve the
> efficiency of creating mappings.
>
> Test results:
> (1) when 20 vCPUs concurrently access 20G ram (all 1G hugepages):
> KVM create block mappings time: 52.83s -> 3.70s
> KVM recover block mappings time(after dirty-logging): 52.0s -> 2.87s
>
> (2) when 40 vCPUs concurrently access 20G ram (all 1G hugepages):
> KVM creating block mappings time: 104.56s -> 3.70s
> KVM recover block mappings time(after dirty-logging): 103.93s -> 2.96s
>
> About patch 2, 3:
> When KVM needs to coalesce the normal page mappings into a block mapping,
> we currently invalidate the old table entry first followed by invalidation
> of TLB, then unmap the page mappings, and install the block entry at last.
>
> It will cost a lot of time to unmap the numerous page mappings, which means
> the table entry will be left invalid for a long time before installation of
> the block entry, and this will cause many spurious translation faults.
>
> So let's quickly install the block entry at first to ensure uninterrupted
> memory access of the other vCPUs, and then unmap the page mappings after
> installation. This will reduce most of the time when the table entry is
> invalid, and avoid most of the unnecessary translation faults.
>
> Test results based on patch 1:
> (1) when 20 vCPUs concurrently access 20G ram (all 1G hugepages):
> KVM recover block mappings time(after dirty-logging): 2.87s -> 0.30s
>
> (2) when 40 vCPUs concurrently access 20G ram (all 1G hugepages):
> KVM recover block mappings time(after dirty-logging): 2.96s -> 0.35s
>
> So combined with patch 1, it makes a big difference of KVM creating mappings
> and recovering block mappings with not much code change.
>
> About patch 4:
> A new method to distinguish cases of memcache allocations is introduced.
> By comparing fault_granule and vma_pagesize, cases that require allocations
> from memcache and cases that don't can be distinguished completely.
>
> ---
>
> Details of test results
> platform: HiSilicon Kunpeng920 (FWB not supported)
> host kernel: Linux mainline (v5.11-rc6)
>
> (1) performance change of patch 1
> cmdline: ./kvm_page_table_test -m 4 -t 2 -g 1G -s 20G -v 20
> 	   (20 vcpus, 20G memory, block mappings(granule 1G))
> Before patch: KVM_CREATE_MAPPINGS: 52.8338s 52.8327s 52.8336s 52.8255s 52.8303s
> After  patch: KVM_CREATE_MAPPINGS:  3.7022s  3.7031s  3.7028s  3.7012s  3.7024s
>
> Before patch: KVM_ADJUST_MAPPINGS: 52.0466s 52.0473s 52.0550s 52.0518s 52.0467s
> After  patch: KVM_ADJUST_MAPPINGS:  2.8787s  2.8781s  2.8785s  2.8742s  2.8759s
>
> cmdline: ./kvm_page_table_test -m 4 -t 2 -g 1G -s 20G -v 40
> 	   (40 vcpus, 20G memory, block mappings(granule 1G))
> Before patch: KVM_CREATE_MAPPINGS: 104.560s 104.556s 104.554s 104.556s 104.550s
> After  patch: KVM_CREATE_MAPPINGS:  3.7011s  3.7103s  3.7005s  3.7024s  3.7106s
>
> Before patch: KVM_ADJUST_MAPPINGS: 103.931s 103.936s 103.927s 103.942s 103.927s
> After  patch: KVM_ADJUST_MAPPINGS:  2.9621s  2.9648s  2.9474s  2.9587s  2.9603s
>
> (2) performance change of patch 2, 3(based on patch 1)
> cmdline: ./kvm_page_table_test -m 4 -t 2 -g 1G -s 20G -v 1
> 	   (1 vcpu, 20G memory, block mappings(granule 1G))
> Before patch: KVM_ADJUST_MAPPINGS: 2.8241s 2.8234s 2.8245s 2.8230s 2.8652s
> After  patch: KVM_ADJUST_MAPPINGS: 0.2444s 0.2442s 0.2423s 0.2441s 0.2429s
>
> cmdline: ./kvm_page_table_test -m 4 -t 2 -g 1G -s 20G -v 20
> 	   (20 vcpus, 20G memory, block mappings(granule 1G))
> Before patch: KVM_ADJUST_MAPPINGS: 2.8787s 2.8781s 2.8785s 2.8742s 2.8759s
> After  patch: KVM_ADJUST_MAPPINGS: 0.3008s 0.3004s 0.2974s 0.2917s 0.2900s
>
> cmdline: ./kvm_page_table_test -m 4 -t 2 -g 1G -s 20G -v 40
> 	   (40 vcpus, 20G memory, block mappings(granule 1G))
> Before patch: KVM_ADJUST_MAPPINGS: 2.9621s 2.9648s 2.9474s 2.9587s 2.9603s
> After  patch: KVM_ADJUST_MAPPINGS: 0.3541s 0.3694s 0.3656s 0.3693s 0.3687s
>
> ---
>
> Yanan Wang (4):
>   KVM: arm64: Move the clean of dcache to the map handler
>   KVM: arm64: Add an independent API for coalescing tables
>   KVM: arm64: Install the block entry before unmapping the page mappings
>   KVM: arm64: Distinguish cases of memcache allocations completely
>
>  arch/arm64/include/asm/kvm_mmu.h | 16 -------
>  arch/arm64/kvm/hyp/pgtable.c     | 82 +++++++++++++++++++++-----------
>  arch/arm64/kvm/mmu.c             | 39 ++++++---------
>  3 files changed, 69 insertions(+), 68 deletions(-)
>