Messages in this thread | | | Date | Tue, 4 Jun 2019 10:26:39 +0100 | From | Dave Martin <> | Subject | Re: [PATCH] KVM: arm64: Drop 'const' from argument of vq_present() |
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On Tue, Jun 04, 2019 at 02:25:45PM +0530, Viresh Kumar wrote: > On 04-06-19, 09:43, Catalin Marinas wrote: > > On Tue, Jun 04, 2019 at 10:13:19AM +0530, Viresh Kumar wrote: > > > We currently get following compilation warning: > > > > > > arch/arm64/kvm/guest.c: In function 'set_sve_vls': > > > arch/arm64/kvm/guest.c:262:18: warning: passing argument 1 of 'vq_present' from incompatible pointer type > > > arch/arm64/kvm/guest.c:212:13: note: expected 'const u64 (* const)[8]' but argument is of type 'u64 (*)[8]' > > > > Since the vq_present() function does not modify the vqs array, I don't > > understand why this warning. Compiler bug? > > Probably yes. Also marking array argument to functions as const is a > right thing to do, to declare that the function wouldn't change the > array values. > > I tried a recent toolchain and this doesn't happen anymore. > > Sorry for the noise.
Sparse is already warning about this, but I had dismissed it as a false positive.
I think this is an instance of disallowing implicit conversions of the form
T ** -> T const **
because this allows a const pointer to be silently de-consted, e.g.:
static const T bar;
void foo(T const **p) { *p = &bar; }
T *baz(void) { T *q; foo(&q); return q; }
I _suspect_ that what's going on here is that the compiler is eliminating a level of indirection during inlining (i.e. converting pass-by-reference to direct access, which is precisely what I wanted to happen). This removes the potentially invalid behaviour as a side-effect.
This relies on the compiler optimising / analysing the code aggressively enough though.
So, I don't have a problem with dropping the extra extra const, e.g.:
static bool vq_present( u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS], unsigned int vq)
Since this function is static and only used very locally, I don't see a big risk: the only reason for the extra const was to check that vq_present() doesn't modify vqs when it shouldn't. But it's a trivial function, and the intent is pretty clear without the extra type modifier.
I'm in two minds about whether this is worth fixing, but if you want to post a patch to remove the extra const (or convert vq_present() to a macro), I'll take a look at it.
Cheers ---Dave
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