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

Marc Zyngier
2019-07-04T08:21:04Z

+KVM, Paolo and Radim,

Guys, do you mind picking this single patch and sending it to Linus?
That's the only fix left for 5.2. Alternatively, I can send you a pull
request, but it feels overkill.

Either way, please let me know.

Thanks,

M.

On Wed, 03 Jul 2019 18:42:50 +0100,
Dave Martin <Dave.Martin@arm.com> 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/sve/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/664633.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)
> {
> --
> 2.1.4
>

--
Jazz is not dead, it just smells funny.