Re: [PATCH v2 04/16] KVM: arm64: Optimize host memory aborts

Quentin Perret
2021-07-26T13:13:14Z

On Monday 26 Jul 2021 at 11:35:10 (+0100), Marc Zyngier wrote:
> On Mon, 26 Jul 2021 10:28:53 +0100,
> Quentin Perret <qperret@google.com> wrote:
> > +static int host_stage2_find_range(u64 addr, struct kvm_mem_range *range)
>
> nit: I find 'find_range' a bit odd. We already have found a
> range. We're just trying to narrow it down to something that fits in a
> single block mapping. How about 'host_stage2_adjust_range'?

Ack.

> > +{
> > + u64 granule, start, end;
> > + kvm_pte_t pte;
> > + u32 level;
> > + int ret;
> > +
> > + ret = kvm_pgtable_get_leaf(&host_kvm.pgt, addr, &pte, &level);
> > + if (ret)
> > + return ret;
> > +
> > + if (kvm_pte_valid(pte))
> > + return -EAGAIN;
> > +
> > + if (pte)
> > + return -EPERM;
> > +
> > + do {
> > + granule = kvm_granule_size(level);
> > + start = ALIGN_DOWN(addr, granule);
> > + end = start + granule;
> > + level++;
> > + } while ((level < KVM_PGTABLE_MAX_LEVELS) &&
> > + (!kvm_level_supports_block_mapping(level) ||
> > + start < range->start || range->end < end));
> > +
>
> This expression does my head in. You are trying to find the largest
> block mapping that entirely fits in range, right? Can we just express
> that directly (with a global negation for the purpose of the loop)?
>
> do {
> [...]
> } while (level < KVM_PGTABLE_MAX_LEVELS &&
> !(kvm_level_supports_block_mapping(level) &&
> start >= range->start &&
> end <= range->end));
>
> I personally find this much more readable, because it expresses the
> condition we are looking for rather than a lot of conditions forcing
> us to continue.
>
> You could also use a kvm_mem_range for the iteration, and add a helper
> that checks for the inclusion.

Something like this (untested)?

diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index 75273166d2c5..07d228163090 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -234,9 +234,15 @@ static inline int __host_stage2_idmap(u64 start, u64 end,
__ret; \
})

+static inline bool range_included(struct kvm_mem_range *child,
+ struct kvm_mem_range *parent)
+{
+ return parent->start <= child->start && child->end <= parent->end;
+}
+
static int host_stage2_find_range(u64 addr, struct kvm_mem_range *range)
{
- u64 granule, start, end;
+ struct kvm_mem_range cur;
kvm_pte_t pte;
u32 level;
int ret;
@@ -252,16 +258,15 @@ static int host_stage2_find_range(u64 addr, struct kvm_mem_range *range)
return -EPERM;

do {
- granule = kvm_granule_size(level);
- start = ALIGN_DOWN(addr, granule);
- end = start + granule;
+ u64 granule = kvm_granule_size(level);
+ cur.start = ALIGN_DOWN(addr, granule);
+ cur.end = cur.start + granule;
level++;
} while ((level < KVM_PGTABLE_MAX_LEVELS) &&
- (!kvm_level_supports_block_mapping(level) ||
- start < range->start || range->end < end));
+ !(kvm_level_supports_block_mapping(level) &&
+ range_included(&cur, parent)));

- range->start = start;
- range->end = end;
+ *range = cur;

return 0;
}