Re: [PATCH] KVM: arm64: Limit stage2_apply_range() batch size to 1GB

From: Oliver Upton
Date: Tue Sep 20 2022 - 15:25:41 EST

Hey Ricardo,

On Tue, Sep 20, 2022 at 12:02:08PM -0700, Ricardo Koller wrote:
> On Tue, Sep 20, 2022 at 06:36:29PM +0000, Oliver Upton wrote:
> > Presently stage2_apply_range() works on a batch of memory addressed by a
> > stage 2 root table entry for the VM. Depending on the IPA limit of the
> > VM and PAGE_SIZE of the host, this could address a massive range of
> > memory. Some examples:
> >
> > 4 level, 4K paging -> 512 GB batch size
> >
> > 3 level, 64K paging -> 4TB batch size
> >
> > Unsurprisingly, working on such a large range of memory can lead to soft
> > lockups. When running dirty_log_perf_test:
> >
> > ./dirty_log_perf_test -m -2 -s anonymous_thp -b 4G -v 48
> >
> > watchdog: BUG: soft lockup - CPU#0 stuck for 45s! [dirty_log_perf_:16703]
> > Modules linked in: vfat fat cdc_ether usbnet mii xhci_pci xhci_hcd sha3_generic gq(O)
> > CPU: 0 PID: 16703 Comm: dirty_log_perf_ Tainted: G O 6.0.0-smp-DEV #1
> > pstate: 80400009 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> > pc : dcache_clean_inval_poc+0x24/0x38
> > lr : clean_dcache_guest_page+0x28/0x4c
> > sp : ffff800021763990
> > pmr_save: 000000e0
> > x29: ffff800021763990 x28: 0000000000000005 x27: 0000000000000de0
> > x26: 0000000000000001 x25: 00400830b13bc77f x24: ffffad4f91ead9c0
> > x23: 0000000000000000 x22: ffff8000082ad9c8 x21: 0000fffafa7bc000
> > x20: ffffad4f9066ce50 x19: 0000000000000003 x18: ffffad4f92402000
> > x17: 000000000000011b x16: 000000000000011b x15: 0000000000000124
> > x14: ffff07ff8301d280 x13: 0000000000000000 x12: 00000000ffffffff
> > x11: 0000000000010001 x10: fffffc0000000000 x9 : ffffad4f9069e580
> > x8 : 000000000000000c x7 : 0000000000000000 x6 : 000000000000003f
> > x5 : ffff07ffa2076980 x4 : 0000000000000001 x3 : 000000000000003f
> > x2 : 0000000000000040 x1 : ffff0830313bd000 x0 : ffff0830313bcc40
> > Call trace:
> > dcache_clean_inval_poc+0x24/0x38
> > stage2_unmap_walker+0x138/0x1ec
> > __kvm_pgtable_walk+0x130/0x1d4
> > __kvm_pgtable_walk+0x170/0x1d4
> > __kvm_pgtable_walk+0x170/0x1d4
> > __kvm_pgtable_walk+0x170/0x1d4
> > kvm_pgtable_stage2_unmap+0xc4/0xf8
> > kvm_arch_flush_shadow_memslot+0xa4/0x10c
> > kvm_set_memslot+0xb8/0x454
> > __kvm_set_memory_region+0x194/0x244
> > kvm_vm_ioctl_set_memory_region+0x58/0x7c
> > kvm_vm_ioctl+0x49c/0x560
> > __arm64_sys_ioctl+0x9c/0xd4
> > invoke_syscall+0x4c/0x124
> > el0_svc_common+0xc8/0x194
> > do_el0_svc+0x38/0xc0
> > el0_svc+0x2c/0xa4
> > el0t_64_sync_handler+0x84/0xf0
> > el0t_64_sync+0x1a0/0x1a4
> >
> > Given the various paging configurations used by KVM at stage 2 there
> > isn't a sensible page table level to use as the batch size. Use 1GB as
> > the batch size instead, as it is evenly divisible by all supported
> > hugepage sizes across 4K, 16K, and 64K paging.
> >
> > Signed-off-by: Oliver Upton <oliver.upton@xxxxxxxxx>
> > ---
> >
> > Applies to 6.0-rc3. Tested with 4K and 64K pages with the above
> > dirty_log_perf_test command and noticed no more soft lockups. I don't
> > have a 16K system to test with.
> >
> > Marc, we spoke about this a while ago and agreed to go for some page
> > table level based batching scheme. However, I decided against that
> > because it doesn't really solve the problem for non-4K kernels.
> >
> > arch/arm64/include/asm/stage2_pgtable.h | 20 --------------------
> > arch/arm64/kvm/mmu.c | 8 +++++++-
> > 2 files changed, 7 insertions(+), 21 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/stage2_pgtable.h b/arch/arm64/include/asm/stage2_pgtable.h
> > index fe341a6578c3..c8dca8ae359c 100644
> > --- a/arch/arm64/include/asm/stage2_pgtable.h
> > +++ b/arch/arm64/include/asm/stage2_pgtable.h
> > @@ -10,13 +10,6 @@
> >
> > #include <linux/pgtable.h>
> >
> > -/*
> > - * PGDIR_SHIFT determines the size a top-level page table entry can map
> > - * and depends on the number of levels in the page table. Compute the
> > - * PGDIR_SHIFT for a given number of levels.
> > - */
> > -#define pt_levels_pgdir_shift(lvls) ARM64_HW_PGTABLE_LEVEL_SHIFT(4 - (lvls))
> > -
> > /*
> > * The hardware supports concatenation of up to 16 tables at stage2 entry
> > * level and we use the feature whenever possible, which means we resolve 4
> > @@ -30,11 +23,6 @@
> > #define stage2_pgtable_levels(ipa) ARM64_HW_PGTABLE_LEVELS((ipa) - 4)
> > #define kvm_stage2_levels(kvm) VTCR_EL2_LVLS(kvm->arch.vtcr)
> >
> > -/* stage2_pgdir_shift() is the size mapped by top-level stage2 entry for the VM */
> > -#define stage2_pgdir_shift(kvm) pt_levels_pgdir_shift(kvm_stage2_levels(kvm))
> > -#define stage2_pgdir_size(kvm) (1ULL << stage2_pgdir_shift(kvm))
> > -#define stage2_pgdir_mask(kvm) ~(stage2_pgdir_size(kvm) - 1)
> > -
> > /*
> > * kvm_mmmu_cache_min_pages() is the number of pages required to install
> > * a stage-2 translation. We pre-allocate the entry level page table at
> > @@ -42,12 +30,4 @@
> > */
> > #define kvm_mmu_cache_min_pages(kvm) (kvm_stage2_levels(kvm) - 1)
> >
> > -static inline phys_addr_t
> > -stage2_pgd_addr_end(struct kvm *kvm, phys_addr_t addr, phys_addr_t end)
> > -{
> > - phys_addr_t boundary = (addr + stage2_pgdir_size(kvm)) & stage2_pgdir_mask(kvm);
> > -
> > - return (boundary - 1 < end - 1) ? boundary : end;
> > -}
> > -
> > #endif /* __ARM64_S2_PGTABLE_H_ */
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index c9a13e487187..d64032b9fbb6 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -31,6 +31,12 @@ static phys_addr_t hyp_idmap_vector;
> >
> > static unsigned long io_map_base;
> >
> > +static inline phys_addr_t stage2_apply_range_next(phys_addr_t addr, phys_addr_t end)
> > +{
> > + phys_addr_t boundary = addr + SZ_1G;
>
> I think you want this to be aligned-down to 1G as well. At least
> stage2_pgd_addr_end() was doing so, plus it reduces the number of
> operations (e.g., when splitting a 1GB huge page when the address is
> unaligned).

Doh! Yeah, we'll want to preserve the alignment that was being done. I'll
post v2 here in a few days but I'll include your suggestion.

--
Thanks,
Oliver