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[31.30.174.28]) by smtp.gmail.com with ESMTPSA id g2sm10186162edp.74.2021.10.06.08.23.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 06 Oct 2021 08:23:03 -0700 (PDT) Date: Wed, 6 Oct 2021 17:23:02 +0200 From: Andrew Jones To: Alexandru Elisei Cc: maz@kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu Subject: Re: [PATCH 2/2] KVM: arm64: Use get_raz_reg() for userspace reads of PMSWINC_EL0 Message-ID: <20211006152302.2tmih2vx5x7xipml@gator.home> References: <20210927124911.191729-1-alexandru.elisei@arm.com> <20210927124911.191729-3-alexandru.elisei@arm.com> <20210930132915.cpfzxdlws2zlou47@gator> MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=drjones@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20211006_082309_151585_836F349E X-CRM114-Status: GOOD ( 38.60 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, Oct 06, 2021 at 03:49:19PM +0100, Alexandru Elisei wrote: > Hi Drew, > > Thank you for the review! > > On Thu, Sep 30, 2021 at 03:29:15PM +0200, Andrew Jones wrote: > > On Mon, Sep 27, 2021 at 01:49:11PM +0100, Alexandru Elisei wrote: > > > PMSWINC_EL0 is a write-only register and was initially part of the VCPU > > > register state, but was later removed in commit 7a3ba3095a32 ("KVM: > > > arm64: Remove PMSWINC_EL0 shadow register"). To prevent regressions, the > > > register was kept accessible from userspace as Read-As-Zero (RAZ). > > > > > > The read function that is used to handle userspace reads of this > > > register is get_raz_id_reg(), which, while technically correct, as it > > > returns 0, it is not semantically correct, as PMSWINC_EL0 is not an ID > > > register as the function name suggests. > > > > > > Add a new function, get_raz_reg(), to use it as the accessor for > > > PMSWINC_EL0, as to not conflate get_raz_id_reg() to handle other types > > > of registers. > > > > > > No functional change intended. > > > > > > Signed-off-by: Alexandru Elisei > > > --- > > > arch/arm64/kvm/sys_regs.c | 11 ++++++++++- > > > 1 file changed, 10 insertions(+), 1 deletion(-) > > > > > > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c > > > index 4adda8bf3168..1be827740f87 100644 > > > --- a/arch/arm64/kvm/sys_regs.c > > > +++ b/arch/arm64/kvm/sys_regs.c > > > @@ -1285,6 +1285,15 @@ static int set_raz_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd, > > > return __set_id_reg(vcpu, rd, uaddr, true); > > > } > > > > > > +static int get_raz_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd, > > > + const struct kvm_one_reg *reg, void __user *uaddr) > > > +{ > > > + const u64 id = sys_reg_to_index(rd); > > > + const u64 val = 0; > > > + > > > + return reg_to_user(uaddr, &val, id); > > > +} > > > + > > > static int set_wi_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd, > > > const struct kvm_one_reg *reg, void __user *uaddr) > > > { > > > @@ -1647,7 +1656,7 @@ static const struct sys_reg_desc sys_reg_descs[] = { > > > * previously (and pointlessly) advertised in the past... > > > */ > > > { PMU_SYS_REG(SYS_PMSWINC_EL0), > > > - .get_user = get_raz_id_reg, .set_user = set_wi_reg, > > > + .get_user = get_raz_reg, .set_user = set_wi_reg, > > > .access = access_pmswinc, .reset = NULL }, > > > { PMU_SYS_REG(SYS_PMSELR_EL0), > > > .access = access_pmselr, .reset = reset_pmselr, .reg = PMSELR_EL0 }, > > > -- > > > 2.33.0 > > > > > > > What about replacing get_raz_id_reg() with this new function? Do really need > > both? > > I thought about that when writing this patch. I ultimately decided against it > because changing the get_user accessor to be get_raz_reg() instead of > get_raz_id_reg() would break the symmetry with set_user, which needs to stay > set_raz_id_reg(), and cannot be substituted with set_wi_reg() because that would > be a change in behaviour (set_raz_id_reg() checks that val == 0, set_wi_reg() > doesn't). > > I do agree that get_raz_id_reg() does the exact same thing as get_raz_reg(), but > in a more roundabout manner. So if you still feel that I should use > get_raz_reg() instead, I'll do that for the next iteration of the series. What > do you think? I'd prefer we avoid maintaining two implementations of the same functionality. If we want to keep the symmetry with set_raz_id_reg, then we could implement get_raz_id_reg as 'return get_raz_reg()'. Thanks, drew _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel