RE: [PATCH] KVM: arm64/sve: Fix vq_present() macro to yield a bool

["Zhang, Lei" <zhang.lei@jp.fujitsu.com>]

Hi guys,

I have started up KVM guest os successfully with SVE feature with Dave' patch.

Tested-by: Zhang Lei <zhang.lei@jp.fujitsu.com >

> -----Original Message-----
> From: Dave Martin <Dave.Martin@arm.com>
> Sent: Thursday, July 04, 2019 5:35 PM
> To: Viresh Kumar <viresh.kumar@linaro.org>
> Cc: Okamoto, Takayuki/岡本 高幸 <tokamoto@jp.fujitsu.com>; Christoffer
> Dall <cdall@kernel.org>; Ard Biesheuvel <ard.biesheuvel@linaro.org>; Marc
> Zyngier <marc.zyngier@arm.com>; Catalin Marinas
> <catalin.marinas@arm.com>; Will Deacon <will.deacon@arm.com>; Zhang,
> Lei/張 雷 <zhang.lei@jp.fujitsu.com>; Julien Grall <julien.grall@arm.com>;
> kvmarm@lists.cs.columbia.edu; linux-arm-kernel@lists.infradead.org
> Subject: Re: [PATCH] KVM: arm64/sve: Fix vq_present() macro to yield a bool
> 
> On Thu, Jul 04, 2019 at 08:32:52AM +0530, Viresh Kumar wrote:
> > On 03-07-19, 18:42, Dave Martin wrote:
> > > From: Zhang Lei <zhang.lei@jp.fujitsu.com>
> > >
> > > The original implementation of vq_present() relied on aggressive
> > > inlining in order for the compiler to know that the code is correct,
> > > due to some const-casting issues.  This was causing sparse and clang
> > > to complain, while GCC compiled cleanly.
> > >
> > > Commit 0c529ff789bc addressed this problem, but since vq_present()
> > > is no longer a function, there is now no implicit casting of the
> > > returned value to the return type (bool).
> > >
> > > In set_sve_vls(), this uncast bit value is compared against a bool,
> > > and so may spuriously compare as unequal when both are nonzero.  As
> > > a result, KVM may reject valid SVE vector length configurations as
> > > invalid, and vice versa.
> > >
> > > Fix it by forcing the returned value to a bool.
> > >
> > > Signed-off-by: Zhang Lei <zhang.lei@jp.fujitsu.com>
> > > Fixes: 0c529ff789bc ("KVM: arm64: Implement vq_present() as a
> > > macro")
> > > Signed-off-by: Dave Martin <Dave.Martin@arm.com> [commit message
> > > rewrite]
> > > Cc: Viresh Kumar <viresh.kumar@linaro.org>
> > >
> > > ---
> > >
> > > Posting this under Zhang Lei's authorship, due to the need to turn
> > > this fix around quickly.  The fix is as per the original suggestion [1].
> > >
> > > Originally observed with the QEMU KVM SVE support under review:
> > > https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg04945.html
> > >
> > > Bug reproduced and fix tested on the Arm Fast Model, with
> > > http://linux-arm.org/git?p=kvmtool-dm.git;a=shortlog;h=refs/heads/sv
> > > e/v3/head (After rerunning util/update_headers.sh.)
> > >
> > > (the --sve-vls command line argument was removed in v4 of the
> > > kvmtool patches).
> > >
> > > [1]
> > > http://lists.infradead.org/pipermail/linux-arm-kernel/2019-July/6646
> > > 33.html
> > > ---
> > >  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
> > > c2afa79..dfd6264 100644
> > > --- a/arch/arm64/kvm/guest.c
> > > +++ b/arch/arm64/kvm/guest.c
> > > @@ -208,7 +208,7 @@ static int set_core_reg(struct kvm_vcpu *vcpu,
> > > const struct kvm_one_reg *reg)
> > >
> > >  #define vq_word(vq) (((vq) - SVE_VQ_MIN) / 64)  #define vq_mask(vq)
> > > ((u64)1 << ((vq) - SVE_VQ_MIN) % 64) -#define vq_present(vqs, vq)
> > > ((vqs)[vq_word(vq)] & vq_mask(vq))
> > > +#define vq_present(vqs, vq) (!!((vqs)[vq_word(vq)] & vq_mask(vq)))
> > >
> > >  static int get_sve_vls(struct kvm_vcpu *vcpu, const struct
> > > kvm_one_reg *reg)  {
> >
> > It was a really nice bug :)
> >
> > Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
> 
> Thanks for the quick review!
> 
> Maybe it makes sense to write equality comparisons on bools as !x == !y to be
> more defensive against this kind of thing.  Anyway, probably best to leave this
> as-is while the dust settles.
> 
> Cheers
> ---Dave
_______________________________________________
kvmarm mailing list
kvmarm@lists.cs.columbia.edu
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm