Re: [PATCH v2 2/3] KVM: arm64: Add prejudgement for relaxing permissions only case in stage2 translation fault handler

[wangyanan (Y)]

On 2021/1/13 23:44, Will Deacon wrote:
> 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@xxxxxxxxxx>
> > ---
> >   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.
Ok, I will make some adjustment for this and post the v3 soon after.

Thanks,

Yanan.