Re: [PATCH] KVM: arm64: Drop 'const' from argument of vq_present()
On 04/06/2019 05:43, 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]' Using which toolchain? My GCC 8.3.0 doesn't say anything. Thanks, M. -- Jazz is not dead. It just smells funny...
Re: [PATCH] KVM: arm64: Drop 'const' from argument of vq_present()
On 04-06-19, 09:30, Marc Zyngier wrote: > On 04/06/2019 05:43, 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]' > > Using which toolchain? My GCC 8.3.0 doesn't say anything. I haven't updated mine since a long time it seems :) aarch64-linux-gnu-gcc (Linaro GCC 4.9-2015.05) 4.9.3 20150413 (prerelease) -- viresh ___ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
Re: [PATCH] KVM: arm64: Drop 'const' from argument of vq_present()
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? -- Catalin
Re: [PATCH] KVM: arm64: Drop 'const' from argument of vq_present()
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. -- viresh ___ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
Re: [PATCH] KVM: arm64: Drop 'const' from argument of vq_present()
On 04/06/2019 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? Yeah, I'm a bit puzzled by that one. Also checked with clang, which is usually much more picky about things, but it didn't complain. We could drop the const without much harm, but I really wish we weed out these ancient compilers... Thanks, M. -- Jazz is not dead. It just smells funny... ___ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
Re: [PATCH] KVM: arm64: Drop 'const' from argument of vq_present()
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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Re: [PATCH] KVM: arm64: Drop 'const' from argument of vq_present()
On 04-06-19, 10:26, Dave Martin wrote: > 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. This patch already does what you are asking for (remove the extra const), isn't it ? I looked at my textbook (The C programming Language, By Brian W. Kernighan and Dennis M. Ritchie.) and it says: " The const declaration can also be used with array arguments, to indicate that the function does not change that array: int strlen(const char[]); " and so this patch isn't necessary for sure. -- viresh ___ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
Re: [PATCH] KVM: arm64: Drop 'const' from argument of vq_present()
On Tue, Jun 04, 2019 at 03:01:53PM +0530, Viresh Kumar wrote: > On 04-06-19, 10:26, Dave Martin wrote: > > 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. > > This patch already does what you are asking for (remove the extra > const), isn't it ? Yes, sorry -- I didn't scroll back far enough. > I looked at my textbook (The C programming Language, By Brian W. > Kernighan and Dennis M. Ritchie.) and it says: > > " > The const declaration can also be used with array arguments, to > indicate that the function does not change that array: > > int strlen(const char[]); > " > > and so this patch isn't necessary for sure. This is an array to which a pointer argument points, not an array argument. I think that's how we hit the constified double-indirection problem. Cheers ---Dave
Re: [PATCH] KVM: arm64: Drop 'const' from argument of vq_present()
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]'
>
> The argument can't be const, as it is copied at runtime using
> copy_from_user(). Drop const from the prototype of vq_present().
>
> Fixes: 9033bba4b535 ("KVM: arm64/sve: Add pseudo-register for the guest's
> vector lengths")
> Signed-off-by: Viresh Kumar
> ---
> arch/arm64/kvm/guest.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index 3ae2f82fca46..78f5a4f45e0a 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -209,7 +209,7 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const
> struct kvm_one_reg *reg)
> #define vq_mask(vq) ((u64)1 << ((vq) - SVE_VQ_MIN) % 64)
>
> static bool vq_present(
> - const u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS],
> + u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS],
> unsigned int vq)
> {
> return (*vqs)[vq_word(vq)] & vq_mask(vq);
Ack, but maybe this should just be converted to a macro?
It already feels a bit like overkill for this to be a function.
Cheers
---Dave
Re: [PATCH] KVM: arm64: Drop 'const' from argument of vq_present()
On 04-06-19, 10:59, Dave Martin 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]'
> >
> > The argument can't be const, as it is copied at runtime using
> > copy_from_user(). Drop const from the prototype of vq_present().
> >
> > Fixes: 9033bba4b535 ("KVM: arm64/sve: Add pseudo-register for the guest's
> > vector lengths")
> > Signed-off-by: Viresh Kumar
> > ---
> > arch/arm64/kvm/guest.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> > index 3ae2f82fca46..78f5a4f45e0a 100644
> > --- a/arch/arm64/kvm/guest.c
> > +++ b/arch/arm64/kvm/guest.c
> > @@ -209,7 +209,7 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const
> > struct kvm_one_reg *reg)
> > #define vq_mask(vq) ((u64)1 << ((vq) - SVE_VQ_MIN) % 64)
> >
> > static bool vq_present(
> > - const u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS],
> > + u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS],
> > unsigned int vq)
> > {
> > return (*vqs)[vq_word(vq)] & vq_mask(vq);
>
> Ack, but maybe this should just be converted to a macro?
I will send a patch with that if that's what you want.
Thanks.
--
viresh
Re: [PATCH] KVM: arm64: Drop 'const' from argument of vq_present()
On Fri, Jun 07, 2019 at 11:30:37AM +0530, Viresh Kumar wrote:
> On 04-06-19, 10:59, Dave Martin 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]'
> > >
> > > The argument can't be const, as it is copied at runtime using
> > > copy_from_user(). Drop const from the prototype of vq_present().
> > >
> > > Fixes: 9033bba4b535 ("KVM: arm64/sve: Add pseudo-register for the guest's
> > > vector lengths")
> > > Signed-off-by: Viresh Kumar
> > > ---
> > > arch/arm64/kvm/guest.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> > > index 3ae2f82fca46..78f5a4f45e0a 100644
> > > --- a/arch/arm64/kvm/guest.c
> > > +++ b/arch/arm64/kvm/guest.c
> > > @@ -209,7 +209,7 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const
> > > struct kvm_one_reg *reg)
> > > #define vq_mask(vq) ((u64)1 << ((vq) - SVE_VQ_MIN) % 64)
> > >
> > > static bool vq_present(
> > > - const u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS],
> > > + u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS],
> > > unsigned int vq)
> > > {
> > > return (*vqs)[vq_word(vq)] & vq_mask(vq);
> >
> > Ack, but maybe this should just be converted to a macro?
>
> I will send a patch with that if that's what you want.
I think this would solve the problem and simplify the code a bit at the
same time.
So go for it.
Cheers
---Dave
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