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

From: Dave Martin <Dave.Martin@arm.com>
To: kvmarm@lists.cs.columbia.edu
Cc: Okamoto Takayuki <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 <zhang.lei@jp.fujitsu.com>,
	Julien Grall <julien.grall@arm.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	linux-arm-kernel@lists.infradead.org
Subject: [PATCH] KVM: arm64/sve: Fix vq_present() macro to yield a bool
Date: Wed,  3 Jul 2019 18:42:50 +0100	[thread overview]
Message-ID: <1562175770-10952-1-git-send-email-Dave.Martin@arm.com> (raw)

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

_______________________________________________
kvmarm mailing list
kvmarm@lists.cs.columbia.edu
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
