From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S978745AbdDXWOf (ORCPT ); Mon, 24 Apr 2017 18:14:35 -0400 Received: from mail-by2nam01on0073.outbound.protection.outlook.com ([104.47.34.73]:47603 "EHLO NAM01-BY2-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S977261AbdDXWO2 (ORCPT ); Mon, 24 Apr 2017 18:14:28 -0400 Authentication-Results: amd.com; dkim=none (message not signed) header.d=none;amd.com; dmarc=none action=none header.from=amd.com; Subject: Re: [PATCH] x86: kvm: Avoid guest page table walk when gpa_available is set To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= References: <1493049146-19261-1-git-send-email-brijesh.singh@amd.com> <20170424205236.GE5713@potion> CC: , , , , , , , , , From: Brijesh Singh Message-ID: <77f51978-5937-0c94-13b6-885345921b03@amd.com> Date: Mon, 24 Apr 2017 17:14:21 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: <20170424205236.GE5713@potion> Content-Type: text/plain; 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>> + gpa_t gpa_val; > > Can't we pass this information through function parameters? > > (I'd rather avoid intractable variables.) > I also wanted to avoid adding yet another variable but we can't depend on cr2 parameters passed into x86_emulate_instruction(). The x86_emulate_instruction() function is called from two places: 1) handling the page-fault. pf_interception [svm.c] kvm_mmu_page_fault [mmu.c] x86_emulate_instruction [x86.c] 2) completing the IO/MMIO's from previous instruction decode kvm_arch_vcpu_ioctl_run complete_emulated_io emulate_instruction x86_emulate_instruction(vcpu, 0, emulation_type, NULL, 0) In #1, we are guaranteed that cr2 variable will contain a valid GPA but in #2, CR2 is set to zero. >> }; >> >> struct kvm_lpage_info { >> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c >> index 5fba706..8827e4b 100644 >> --- a/arch/x86/kvm/svm.c >> +++ b/arch/x86/kvm/svm.c >> @@ -4159,6 +4159,10 @@ static int handle_exit(struct kvm_vcpu *vcpu) >> >> vcpu->arch.gpa_available = (exit_code == SVM_EXIT_NPF); >> >> + /* On #NPF, exit_info_2 contain a valid GPA */ >> + if (vcpu->arch.gpa_available) >> + vcpu->arch.gpa_val = svm->vmcb->control.exit_info_2; > > How is vcpu->arch.gpa_val used between here and the NPF handler? > handle_exit [svm.c] pf_interception [svm.c] /* it invokes the fault handler with CR2 = svm->vmcb->control.exit_info_2 */ kvm_mmu_page_fault [mmu.c] x86_emulate_instruction [x86.c] emulator_read_write_onepage [x86.c] /* *this is where we walk the guest page table to translate * a GVA to GPA. If gpa_available is set then we use the * gpa_val instead of walking the pgtable. */ >> + >> if (!is_cr_intercept(svm, INTERCEPT_CR0_WRITE)) >> vcpu->arch.cr0 = svm->vmcb->save.cr0; >> if (npt_enabled) >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> @@ -4653,18 +4653,16 @@ static int emulator_read_write_onepage(unsigned long addr, void *val, >> * occurred. >> */ >> if (vcpu->arch.gpa_available && >> - emulator_can_use_gpa(ctxt) && >> - vcpu_is_mmio_gpa(vcpu, addr, exception->address, write) && >> - (addr & ~PAGE_MASK) == (exception->address & ~PAGE_MASK)) { >> + emulator_can_use_gpa(ctxt) && >> + (addr & ~PAGE_MASK) == (exception->address & ~PAGE_MASK)) { >> gpa = exception->address; >> - goto mmio; >> + ret = vcpu_is_mmio_gpa(vcpu, addr, gpa, write); >> + } else { >> + ret = vcpu_mmio_gva_to_gpa(vcpu, addr, &gpa, exception, write); >> + if (ret < 0) >> + return X86EMUL_PROPAGATE_FAULT; >> } >> >> - ret = vcpu_mmio_gva_to_gpa(vcpu, addr, &gpa, exception, write); >> - >> - if (ret < 0) >> - return X86EMUL_PROPAGATE_FAULT; >> - >> /* For APIC access vmexit */ >> if (ret) >> goto mmio; >> @@ -5675,8 +5673,14 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, >> } >> >> restart: >> - /* Save the faulting GPA (cr2) in the address field */ >> - ctxt->exception.address = cr2; >> + /* >> + * Save the faulting GPA (cr2) in the address field >> + * NOTE: If gpa_available is set then gpa_val will contain a valid GPA >> + */ >> + if (vcpu->arch.gpa_available) >> + ctxt->exception.address = vcpu->arch.gpa_val; >> + else >> + ctxt->exception.address = cr2; > > And related, shouldn't vcpu->arch.gpa_val be in cr2? > See my previous comment. In some cases CR2 may be set to zero (e.g when completing the instruction from previous io/mmio page-fault). If we are decide to add the gpa_val then we can remove above if statement from x86_emulate_instruction() and update emulator_read_write_onepage to use the vcpu->arch.gpa_val instead of exception->address. if (vcpu->arch.gpa_available && emulator_can_use_gpa(ctxt) && (addr & ~PAGE_MASK) == (exception->address & ~PAGE_MASK)) { gpa = vcpu=>arch.gpa_val; ... ... } -Brijesh