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[34.68.225.194]) by smtp.gmail.com with ESMTPSA id a13-20020a056e02180d00b002c25b51d5ecsm2014992ilv.55.2022.02.25.10.26.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 25 Feb 2022 10:26:38 -0800 (PST) Date: Fri, 25 Feb 2022 18:26:35 +0000 From: Oliver Upton To: Raghavendra Rao Ananta Cc: Marc Zyngier , Andrew Jones , James Morse , Alexandru Elisei , Suzuki K Poulose , Paolo Bonzini , Catalin Marinas , Will Deacon , Peter Shier , Ricardo Koller , Reiji Watanabe , Jing Zhang , linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH v4 02/13] KVM: arm64: Introduce KVM_CAP_ARM_REG_SCOPE Message-ID: References: <20220224172559.4170192-1-rananta@google.com> <20220224172559.4170192-3-rananta@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Spam-Status: No, score=-9.5 required=5.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RDNS_NONE,SPF_HELO_NONE,T_SCC_BODY_TEXT_LINE, USER_IN_DEF_DKIM_WL autolearn=no autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 25, 2022 at 09:34:35AM -0800, Raghavendra Rao Ananta wrote: > Hey Oliver, > > On Thu, Feb 24, 2022 at 10:43 PM Oliver Upton wrote: > > > > On Thu, Feb 24, 2022 at 05:25:48PM +0000, Raghavendra Rao Ananta wrote: > > > KVM_[GET|SET]_ONE_REG act on per-vCPU basis. Currently certain > > > ARM64 registers, such as KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_[1|2], > > > are accessed via this interface even though the effect that > > > they have are really per-VM. As a result, userspace could just > > > waste cycles to read/write the same information for every vCPU > > > that it spawns, only to realize that there's absolutely no change > > > in the VM's state. The problem gets worse in proportion to the > > > number of vCPUs created. > > > > > > As a result, to avoid this redundancy, introduce the capability > > > KVM_CAP_ARM_REG_SCOPE. If enabled, KVM_GET_REG_LIST will advertise > > > the registers that are VM-scoped by dynamically modifying the > > > register encoding. KVM_REG_ARM_SCOPE_* helper macros are introduced > > > to decode the same. By learning this, userspace can access such > > > registers only once. > > > > > > Signed-off-by: Raghavendra Rao Ananta > > > --- > > > Documentation/virt/kvm/api.rst | 16 ++++++++++++++++ > > > arch/arm64/include/asm/kvm_host.h | 3 +++ > > > arch/arm64/include/uapi/asm/kvm.h | 6 ++++++ > > > arch/arm64/kvm/arm.c | 13 +++++++------ > > > include/uapi/linux/kvm.h | 1 + > > > 5 files changed, 33 insertions(+), 6 deletions(-) > > > > > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > > > index a4267104db50..7e7b3439f540 100644 > > > --- a/Documentation/virt/kvm/api.rst > > > +++ b/Documentation/virt/kvm/api.rst > > > @@ -7561,3 +7561,19 @@ The argument to KVM_ENABLE_CAP is also a bitmask, and must be a subset > > > of the result of KVM_CHECK_EXTENSION. KVM will forward to userspace > > > the hypercalls whose corresponding bit is in the argument, and return > > > ENOSYS for the others. > > > + > > > +8.34 KVM_CAP_ARM_REG_SCOPE > > > +-------------------------- > > > + > > > +:Architectures: arm64 > > > + > > > +The capability, if enabled, amends the existing register encoding > > > +with additional information to the userspace if a particular register > > > +is scoped per-vCPU or per-VM via KVM_GET_REG_LIST. KVM provides > > > +KVM_REG_ARM_SCOPE_* helper macros to decode the same. Userspace can > > > +use this information from the register encoding to access a VM-scopped > > > +regiser only once, as opposed to accessing it for every vCPU for the > > > +same effect. > > > + > > > > Could you describe the encoding changes in 4.68 'KVM_SET_ONE_REG', along > > with the other ARM encoding details? > > > > > +On the other hand, if the capability is disabled, all the registers > > > +remain vCPU-scopped by default, retaining backward compatibility. > > > > typo: vCPU-scoped > > > > That said, I don't believe we need to document behavior if the CAP is > > disabled, as the implicated ioctls should continue to work the same. > > > Sure, I'll address the above two Doc comments. > > > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h > > > index 5bc01e62c08a..8132de6bd718 100644 > > > --- a/arch/arm64/include/asm/kvm_host.h > > > +++ b/arch/arm64/include/asm/kvm_host.h > > > @@ -136,6 +136,9 @@ struct kvm_arch { > > > > > > /* Memory Tagging Extension enabled for the guest */ > > > bool mte_enabled; > > > + > > > + /* Register scoping enabled for KVM registers */ > > > + bool reg_scope_enabled; > > > }; > > > > > > struct kvm_vcpu_fault_info { > > > diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h > > > index b3edde68bc3e..c35447cc0e0c 100644 > > > --- a/arch/arm64/include/uapi/asm/kvm.h > > > +++ b/arch/arm64/include/uapi/asm/kvm.h > > > @@ -199,6 +199,12 @@ struct kvm_arm_copy_mte_tags { > > > #define KVM_REG_ARM_COPROC_MASK 0x000000000FFF0000 > > > #define KVM_REG_ARM_COPROC_SHIFT 16 > > > > > > +/* Defines if a KVM register is one per-vCPU or one per-VM */ > > > +#define KVM_REG_ARM_SCOPE_MASK 0x0000000010000000 > > > +#define KVM_REG_ARM_SCOPE_SHIFT 28 > > > > Thinking about the advertisement of VM- and vCPU-scoped registers, this > > could be generally useful. Might it make sense to add such an encoding > > to the arch-generic register definitions? > > > > If that is the case, we may want to snap up a few more bits (a nybble) > > for future expansion. > > > That's a great idea! But I wonder if we'll get a push-back since there > are no users of it in other arch(s) yet. Not sure if there was any > need/discussion regarding the same, but I'm happy to share a patch for > the same if you sense that there's a strong potential for the patch. > I'm unsure if this is actually of interest to other architectures, it just doesn't seem ARM-specific so we should probably raise the question so we only grab these bits once. > > > +#define KVM_REG_ARM_SCOPE_VCPU 0 > > > +#define KVM_REG_ARM_SCOPE_VM 1 > > > + > > > /* Normal registers are mapped as coprocessor 16. */ > > > #define KVM_REG_ARM_CORE (0x0010 << KVM_REG_ARM_COPROC_SHIFT) > > > #define KVM_REG_ARM_CORE_REG(name) (offsetof(struct kvm_regs, name) / sizeof(__u32)) > > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c > > > index ecc5958e27fe..107977c82c6c 100644 > > > --- a/arch/arm64/kvm/arm.c > > > +++ b/arch/arm64/kvm/arm.c > > > @@ -81,26 +81,26 @@ int kvm_arch_check_processor_compat(void *opaque) > > > int kvm_vm_ioctl_enable_cap(struct kvm *kvm, > > > struct kvm_enable_cap *cap) > > > { > > > - int r; > > > + int r = 0; > > > > > > if (cap->flags) > > > return -EINVAL; > > > > > > switch (cap->cap) { > > > case KVM_CAP_ARM_NISV_TO_USER: > > > - r = 0; > > > kvm->arch.return_nisv_io_abort_to_user = true; > > > break; > > > case KVM_CAP_ARM_MTE: > > > mutex_lock(&kvm->lock); > > > - if (!system_supports_mte() || kvm->created_vcpus) { > > > + if (!system_supports_mte() || kvm->created_vcpus) > > > r = -EINVAL; > > > - } else { > > > - r = 0; > > > + else > > > kvm->arch.mte_enabled = true; > > > - } > > > mutex_unlock(&kvm->lock); > > > break; > > > > Hmm.. these all look like cleanups. If you want to propose these, could > > you do it in a separate patch? > > > Ahh, I thought I could squeeze it in. But sure, I can separate it out. > > > + case KVM_CAP_ARM_REG_SCOPE: > > > + WRITE_ONCE(kvm->arch.reg_scope_enabled, true); > > > + break; > > > default: > > > r = -EINVAL; > > > break; > > > @@ -209,6 +209,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) > > > case KVM_CAP_SET_GUEST_DEBUG: > > > case KVM_CAP_VCPU_ATTRIBUTES: > > > case KVM_CAP_PTP_KVM: > > > + case KVM_CAP_ARM_REG_SCOPE: > > > > It is a bit odd to advertise a capability (and allow userspace to enable > > it), despite the fact that the feature itself hasn't yet been > > implemented. > > > > Is it possible to fold the feature in to the patch that exposes it to > > userspace? Otherwise, you could punt advertisement of the CAP until it > > is actually implemented in kernel. > > > Well, I didn't want to complicate the patch, but technically the > feature is available with this patch, including all the CAP and macro > definitions. Userspace can still decode the scope information, only > that no registers are added yet, which is done in the next patch. So, > the userspace can still remain the same between this and the next > patch. But the series isn't cleanly bisectable. There will exist commits in history that report KVM_CAP_ARM_REG_SCOPE as implemented even though that is not actually the case. You should really only advertise support to userspace when the feature is implemented. Defining kvm->arch.reg_scope_enabled can be done earlier so you have a bit to test and guard all of the new code, and only expose the CAP in the last patch of the series. Also, as an FYI Marc has a patch that I'll be picking up in my own series which uses bits instead of bools to keep track of certain VM-wide features: https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/commit/?h=kvm-arm64/mmu/guest-MMIO-guard&id=7dd0a13a4217b870f2e83cdc6045e5ce482a5340 Marc, if neither of our series land in 5.18 could you at least submit this patch in preparation? Should keep conflicts minimal that way. Thanks! -- Oliver