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

From: Paolo Bonzini <pbonzini@redhat.com>
To: "Marc Zyngier" <marc.zyngier@arm.com>,
	"Dave Martin" <Dave.Martin@arm.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>
Cc: Okamoto Takayuki <tokamoto@jp.fujitsu.com>,
	Christoffer Dall <cdall@kernel.org>, kvm <kvm@vger.kernel.org>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	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>,
	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
Date: Thu, 4 Jul 2019 14:24:42 +0200	[thread overview]
Message-ID: <1f39cc48-12d5-2e56-c218-b6b0dd05d8ce@redhat.com> (raw)
In-Reply-To: <86wogynrbt.wl-marc.zyngier@arm.com>

On 04/07/19 10:20, Marc Zyngier wrote:
> +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.

Sure, will do.

Paolo

> 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
>>
> 

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