Date:   Tue, 27 Sep 2022 07:34:22 -0400
From:   Marc Zyngier <maz@...nel.org>
To:     Oliver Upton <oliver.upton@...ux.dev>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Ricardo Koller <ricarkol@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH v2] KVM: arm64: Limit stage2_apply_range() batch size to 1GB

On Mon, 26 Sep 2022 18:21:45 -0400,
Oliver Upton <oliver.upton@...ux.dev> 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@...ux.dev>
> ---
> 
> Applies to 6.0-rc3. Tested with 4K, 16K, and 64K pages with the above
> dirty_log_perf_test command and noticed no more soft lockups.
> 
> v1: https://lore.kernel.org/kvmarm/20220920183630.3376939-1-oliver.upton@linux.dev/
> 
> v1 -> v2:
>  - Align down to the next 1GB boundary (Ricardo)
> 
>  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..5d05bb92e129 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)

Please drop the inline. I'm sure the compiler will perform its
magic.

Can I also bikeshed a bit about the name? This doesn't "apply"
anything, nor does it return the next range. It really computes the
end of the current one.

Something like stage2_range_addr_end() would at least be consistent
with the rest of the arm64 code (grep for _addr_end ...).

> +{
> +	phys_addr_t boundary = round_down(addr + SZ_1G, SZ_1G);

nit: the rest of the code is using ALIGN_DOWN(). Any reason why this
can't be used here?

> +
> +	return (boundary - 1 < end - 1) ? boundary : end;
> +}
>  
>  /*
>   * Release kvm_mmu_lock periodically if the memory region is large. Otherwise,
> @@ -52,7 +58,7 @@ static int stage2_apply_range(struct kvm *kvm, phys_addr_t addr,
>  		if (!pgt)
>  			return -EINVAL;
>  
> -		next = stage2_pgd_addr_end(kvm, addr, end);
> +		next = stage2_apply_range_next(addr, end);
>  		ret = fn(pgt, addr, next - addr);
>  		if (ret)
>  			break;
> 

The main problem I see with this is that some entries now get visited
multiple times if they cover more than a single 1GB entry (like a
512GB level-0 entry with 4k pages and 48bit IPA) . As long as this
isn't destructive (CMOs, for example), this is probably OK. For
operations that are not idempotent (such as stage2_unmap_walker), this
is a significant change in behaviour.

My concern is that we have on one side a walker that is strictly
driven by the page-table sizes, and we now get an arbitrary value that
doesn't necessarily a multiple of block sizes. Yes, this works right
now because you can't create a block mapping larger than 1GB with any
of the supported page size.

But with 52bit VA/PA support, this changes: we can have 512GB (4k),
64GB (16k) and 4TB (64k) block mappings at S2. We don't support this
yet at S2, but when this hits, we'll be in potential trouble.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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