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

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Message-ID: <YP604j6B1pkhAnT1@google.com>
Date:   Mon, 26 Jul 2021 14:13:06 +0100
From:   Quentin Perret <qperret@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     james.morse@....com, alexandru.elisei@....com,
        suzuki.poulose@....com, catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, ardb@...nel.org, qwandor@...gle.com,
        tabba@...gle.com, dbrazdil@...gle.com, kernel-team@...roid.com
Subject: Re: [PATCH v2 04/16] KVM: arm64: Optimize host memory aborts

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@...gle.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;
 }