From: Will Deacon <will@kernel.org>
To: Yanan Wang <wangyanan55@huawei.com>
Cc: linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
Marc Zyngier <maz@kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>,
James Morse <james.morse@arm.com>,
Julien Thierry <julien.thierry.kdev@gmail.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
Gavin Shan <gshan@redhat.com>,
Quentin Perret <qperret@google.com>,
wanghaibin.wang@huawei.com, yezengruan@huawei.com,
zhukeqian1@huawei.com, yuzenghui@huawei.com
Subject: Re: [PATCH v2 2/3] KVM: arm64: Add prejudgement for relaxing permissions only case in stage2 translation fault handler
Date: Wed, 13 Jan 2021 15:44:15 +0000 [thread overview]
Message-ID: <20210113154414.GA11892@willie-the-truck> (raw)
In-Reply-To: <20201216122844.25092-3-wangyanan55@huawei.com>
On Wed, Dec 16, 2020 at 08:28:43PM +0800, Yanan Wang wrote:
> In dirty-logging, or dirty-logging-stopped time, even normal running
> time of a guest configed with huge mappings and numbers of vCPUs,
> translation faults by different vCPUs on the same GPA could occur
> successively almost at the same time. There are two reasons for it.
>
> (1) If there are some vCPUs accessing the same GPA at the same time and
> the leaf PTE is not set yet, then they will all cause translation faults
> and the first vCPU holding mmu_lock will set valid leaf PTE, and the
> others will later update the old PTE with a new one if they are different.
>
> (2) When changing a leaf entry or a table entry with break-before-make,
> if there are some vCPUs accessing the same GPA just catch the moment when
> the target PTE is set invalid in a BBM procedure coincidentally, they will
> all cause translation faults and will later update the old PTE with a new
> one if they are different.
>
> The worst case can be like this: vCPU A causes a translation fault with RW
> prot and sets the leaf PTE with RW permissions, and then the next vCPU B
> with RO prot updates the PTE back to RO permissions with break-before-make.
> And the BBM-invalid moment may trigger more unnecessary translation faults,
> then some useless small loops might occur which could lead to vCPU stuck.
>
> To avoid unnecessary update and small loops, add prejudgement in the
> translation fault handler: Skip updating the PTE with break-before-make
> if we are trying to recreate the exact same mapping or only change the
> access permissions. Actually, change of permissions will be handled
> through the relax_perms path next time if necessary.
>
> Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
> ---
> arch/arm64/kvm/hyp/pgtable.c | 28 +++++++++++++++++++---------
> 1 file changed, 19 insertions(+), 9 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 350f9f810930..8225ced49bad 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -45,6 +45,10 @@
>
> #define KVM_PTE_LEAF_ATTR_HI_S2_XN BIT(54)
>
> +#define KVM_PTE_LEAF_ATTR_S2_PERMS (KVM_PTE_LEAF_ATTR_LO_S2_S2AP_R | \
> + KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W | \
> + KVM_PTE_LEAF_ATTR_HI_S2_XN)
> +
> struct kvm_pgtable_walk_data {
> struct kvm_pgtable *pgt;
> struct kvm_pgtable_walker *walker;
> @@ -460,7 +464,7 @@ static int stage2_map_set_prot_attr(enum kvm_pgtable_prot prot,
> return 0;
> }
>
> -static bool stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> +static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> kvm_pte_t *ptep,
> struct stage2_map_data *data)
> {
> @@ -469,13 +473,18 @@ static bool stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> struct page *page = virt_to_page(ptep);
>
> if (!kvm_block_mapping_supported(addr, end, phys, level))
> - return false;
> + return 1;
It would probably be cleaner to return another error code here, as we
have failed to install a mapping (e.g. E2BIG or perhaps more perversely,
ENOTBLK). Then the caller can decide to install a trable.
Will
next prev parent reply other threads:[~2021-01-13 15:45 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-16 12:28 [PATCH v2 0/3] RFC: Solve several problems in stage 2 translation Yanan Wang
2020-12-16 12:28 ` [PATCH v2 1/3] KVM: arm64: Decouple partial code of hyp stage 1 mapping and guest stage 2 mapping Yanan Wang
2020-12-16 12:28 ` [PATCH v2 2/3] KVM: arm64: Add prejudgement for relaxing permissions only case in stage2 translation fault handler Yanan Wang
2021-01-13 15:44 ` Will Deacon [this message]
2021-01-14 9:28 ` wangyanan (Y)
2020-12-16 12:28 ` [PATCH v2 3/3] KVM: arm64: Mark the page dirty only if the fault is handled successfully Yanan Wang
2021-01-13 15:51 ` Will Deacon
2021-01-14 9:28 ` wangyanan (Y)
2020-12-28 2:46 ` [PATCH v2 0/3] RFC: Solve several problems in stage 2 translation wangyanan (Y)
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