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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id k12sm5638463edo.50.2021.04.02.10.07.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 02 Apr 2021 10:07:34 -0700 (PDT) Subject: Re: [PATCH v2 6/9] KVM: x86: implement KVM_GUESTDBG_BLOCKEVENTS To: Maxim Levitsky , kvm@vger.kernel.org Cc: "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , Ingo Molnar , Andrew Morton , Thomas Gleixner , Sean Christopherson , Marc Zyngier , Catalin Marinas , open list , Julien Thierry , Stefano Garzarella , Borislav Petkov , Suzuki K Poulose , Jonathan Corbet , Jessica Yu , "H. Peter Anvin" , Jan Kiszka , Will Deacon , "open list:KERNEL VIRTUAL MACHINE FOR ARM64 (KVM/arm64)" , Vitaly Kuznetsov , Vasily Gorbik , Joerg Roedel , Claudio Imbrenda , Jim Mattson , Cornelia Huck , David Hildenbrand , Wanpeng Li , Janosch Frank , Christian Borntraeger , "open list:S390" , Heiko Carstens , Kieran Bingham , "open list:DOCUMENTATION" , "moderated list:KERNEL VIRTUAL MACHINE FOR ARM64 (KVM/arm64)" , James Morse References: <20210401135451.1004564-1-mlevitsk@redhat.com> <20210401135451.1004564-7-mlevitsk@redhat.com> From: Paolo Bonzini Message-ID: Date: Fri, 2 Apr 2021 19:07:32 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: <20210401135451.1004564-7-mlevitsk@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/04/21 15:54, Maxim Levitsky wrote: > KVM_GUESTDBG_BLOCKEVENTS is a guest debug feature that > will allow KVM to block all interrupts while running. > It is mostly intended to be used together with single stepping, > to make it more robust, and has the following benefits: > > * Resuming from a breakpoint is much more reliable: > When resuming execution from a breakpoint, with interrupts enabled, > more often than not, KVM would inject an interrupt and make the CPU > jump immediately to the interrupt handler and eventually return to > the breakpoint, only to trigger it again. > > From the gdb's user point of view it looks like the CPU has never > executed a single instruction and in some cases that can even > prevent forward progress, for example, when the breakpoint > is placed by an automated script (e.g lx-symbols), which does > something in response to the breakpoint and then continues > the guest automatically. > If the script execution takes enough time for another interrupt to > arrive, the guest will be stuck on the same breakpoint forever. > > * Normal single stepping is much more predictable, since it won't > land the debugger into an interrupt handler. > > * Chances of RFLAGS.TF being leaked to the guest are reduced: > > KVM sets that flag behind the guest's back to single step it, > but if the single step lands the vCPU into an > interrupt/exception handler the RFLAGS.TF will be leaked to the > guest in the form of being pushed to the stack. > This doesn't completely eliminate this problem as exceptions > can still happen, but at least this eliminates the common > case. > > Signed-off-by: Maxim Levitsky The patch uses BLOCKIRQ instead of BLOCKEVENTS. Paolo > --- > Documentation/virt/kvm/api.rst | 1 + > arch/x86/include/asm/kvm_host.h | 3 ++- > arch/x86/include/uapi/asm/kvm.h | 1 + > arch/x86/kvm/x86.c | 4 ++++ > 4 files changed, 8 insertions(+), 1 deletion(-) > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > index 9778b2434c03..a4f2dc84741f 100644 > --- a/Documentation/virt/kvm/api.rst > +++ b/Documentation/virt/kvm/api.rst > @@ -3338,6 +3338,7 @@ flags which can include the following: > - KVM_GUESTDBG_INJECT_DB: inject DB type exception [x86] > - KVM_GUESTDBG_INJECT_BP: inject BP type exception [x86] > - KVM_GUESTDBG_EXIT_PENDING: trigger an immediate guest exit [s390] > + - KVM_GUESTDBG_BLOCKIRQ: avoid injecting interrupts/NMI/SMI [x86] > > For example KVM_GUESTDBG_USE_SW_BP indicates that software breakpoints > are enabled in memory so we need to ensure breakpoint exceptions are > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > index cc7c82a449d5..8c529ae9dbbe 100644 > --- a/arch/x86/include/asm/kvm_host.h > +++ b/arch/x86/include/asm/kvm_host.h > @@ -227,7 +227,8 @@ enum x86_intercept_stage; > KVM_GUESTDBG_USE_HW_BP | \ > KVM_GUESTDBG_USE_SW_BP | \ > KVM_GUESTDBG_INJECT_BP | \ > - KVM_GUESTDBG_INJECT_DB) > + KVM_GUESTDBG_INJECT_DB | \ > + KVM_GUESTDBG_BLOCKIRQ) > > > #define PFERR_PRESENT_BIT 0 > diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h > index 5a3022c8af82..b0f9945067f7 100644 > --- a/arch/x86/include/uapi/asm/kvm.h > +++ b/arch/x86/include/uapi/asm/kvm.h > @@ -282,6 +282,7 @@ struct kvm_debug_exit_arch { > #define KVM_GUESTDBG_USE_HW_BP 0x00020000 > #define KVM_GUESTDBG_INJECT_DB 0x00040000 > #define KVM_GUESTDBG_INJECT_BP 0x00080000 > +#define KVM_GUESTDBG_BLOCKIRQ 0x00100000 > > /* for KVM_SET_GUEST_DEBUG */ > struct kvm_guest_debug_arch { > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 956e8e0bd6af..3627ce8fe5bb 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -8460,6 +8460,10 @@ static void inject_pending_event(struct kvm_vcpu *vcpu, bool *req_immediate_exit > can_inject = false; > } > > + /* Don't inject interrupts if the user asked to avoid doing so */ > + if (vcpu->guest_debug & KVM_GUESTDBG_BLOCKIRQ) > + return; > + > /* > * Finally, inject interrupt events. If an event cannot be injected > * due to architectural conditions (e.g. IF=0) a window-open exit >