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AJvYcCWxtD9m+UnQoo41t/Fk8a3+d2OW84EddrdoejvZZvEDpE7RkSY3Qbjac+MKokNQP3IBVwGNIh9N54BETUNByZCuXorj X-Gm-Message-State: AOJu0YyMR/a57XJMuvj27xJRRg8PXL7LzLYdplyuJv4H4S70l+g5YNcX DIHgeNL1BLFTieUgPdJfTyONo7JvFK/2hsUmkWbCbzCDeCVAcN0E4uemKqcb9A0wC5svd1JJ6tf JJw== X-Google-Smtp-Source: AGHT+IErM1oyl6dX79PLFBbS0Ue2cTuqm2deVMm1K7Fx1UQZ6ahMkuyTT3QbjPRcQErn+dpEqRps5VUDPf4= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a81:770a:0:b0:64a:d9c2:42c1 with SMTP id 00721157ae682-6ac9913772fmr296287b3.5.1723599100911; Tue, 13 Aug 2024 18:31:40 -0700 (PDT) Date: Tue, 13 Aug 2024 18:31:39 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240813051256.2246612-1-binbin.wu@linux.intel.com> <20240813051256.2246612-2-binbin.wu@linux.intel.com> Message-ID: Subject: Re: [PATCH v2 1/2] KVM: x86: Check hypercall's exit to userspace generically From: Sean Christopherson To: Isaku Yamahata Cc: Kai Huang , Binbin Wu , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, pbonzini@redhat.com, rick.p.edgecombe@intel.com, michael.roth@amd.com Content-Type: text/plain; charset="us-ascii" On Tue, Aug 13, 2024, Isaku Yamahata wrote: > On Wed, Aug 14, 2024 at 11:11:29AM +1200, > Kai Huang wrote: > > > > > > > On 14/08/2024 5:50 am, Isaku Yamahata wrote: > > > On Tue, Aug 13, 2024 at 01:12:55PM +0800, > > > Binbin Wu wrote: > > > > > > > Check whether a KVM hypercall needs to exit to userspace or not based on > > > > hypercall_exit_enabled field of struct kvm_arch. > > > > > > > > Userspace can request a hypercall to exit to userspace for handling by > > > > enable KVM_CAP_EXIT_HYPERCALL and the enabled hypercall will be set in > > > > hypercall_exit_enabled. Make the check code generic based on it. > > > > > > > > Signed-off-by: Binbin Wu > > > > --- > > > > arch/x86/kvm/x86.c | 4 ++-- > > > > arch/x86/kvm/x86.h | 7 +++++++ > > > > 2 files changed, 9 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > > > index af6c8cf6a37a..6e16c9751af7 100644 > > > > --- a/arch/x86/kvm/x86.c > > > > +++ b/arch/x86/kvm/x86.c > > > > @@ -10226,8 +10226,8 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu) > > > > cpl = kvm_x86_call(get_cpl)(vcpu); > > > > ret = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl); > > > > - if (nr == KVM_HC_MAP_GPA_RANGE && !ret) > > > > - /* MAP_GPA tosses the request to the user space. */ > > > > + if (!ret && is_kvm_hc_exit_enabled(vcpu->kvm, nr)) > > > > + /* The hypercall is requested to exit to userspace. */ > > > > return 0; > > > > if (!op_64_bit) > > > > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h > > > > index 50596f6f8320..0cbec76b42e6 100644 > > > > --- a/arch/x86/kvm/x86.h > > > > +++ b/arch/x86/kvm/x86.h > > > > @@ -547,4 +547,11 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size, > > > > unsigned int port, void *data, unsigned int count, > > > > int in); > > > > +static inline bool is_kvm_hc_exit_enabled(struct kvm *kvm, unsigned long hc_nr) I would rather have "hypercall" in the name, "hc" never jumps out to me as being "hypercall". Maybe is_hypercall_exit_enabled(), user_exit_on_hypercall(), or just exit_on_hypercall()? I'd probably vote for user_exit_on_hypercall(), as that clarifies it's all about exiting to userspace, not from the guest. > > > > +{ > > > > + if(WARN_ON_ONCE(hc_nr >= sizeof(kvm->arch.hypercall_exit_enabled) * 8)) > > > > + return false; > > > > > > Is this to detect potential bug? Maybe > > > BUILD_BUG_ON(__builtin_constant_p(hc_nr) && > > > !(BIT(hc_nr) & KVM_EXIT_HYPERCALL_VALID_MASK)); > > > Overkill? > > > > I don't think this is the correct way to use __builtin_constant_p(), i.e. it > > doesn't make sense to use __builtin_constant_p() in BUILD_BUG_ON(). KVM does use __builtin_constant_p() to effectively disable some assertions when it's allowed (by KVM's arbitrary rules) to pass in a non-constant value. E.g. see all the vmcs_checkNN() helpers. If we didn't waive the assertion for values that aren't constant at compile-time, all of the segmentation code would need to be unwound into switch statements. But for things like guest_cpuid_has(), the rule is that the input must be a compile-time constant. > > IIUC you need some build time guarantee here, but __builtin_constant_p() can > > return false, in which case the above BUILD_BUG_ON() does nothing, which > > defeats the purpose. > > It depends on what we'd like to detect. BUILT_BUG_ON(__builtin_constant_p()) > can detect the usage in the patch 2/2, > is_kvm_hc_exit_enabled(vcpu->kvm, KVM_HC_MAP_GPA_RANGE). The potential > future use of is_kvm_hc_exit_enabled(, KVM_HC_MAP_future_hypercall). > > Although this version doesn't help for the one in kvm_emulate_hypercall(), > !ret check is done first to avoid WARN_ON_ONCE() to hit here. > > Maybe we can just drop this WARN_ON_ONCE(). Yeah, I think it makes sense to drop the WARN, otherwise I suspect we'll end up dancing around the helper just to avoid the warning. I'm 50/50 on the BUILD_BUG_ON(). One one hand, it's kinda overkill. On the other hand, it's zero generated code.