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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id m3sm3548604edc.36.2021.10.29.05.56.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 29 Oct 2021 05:56:06 -0700 (PDT) From: Vitaly Kuznetsov To: Sean Christopherson Cc: Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, llvm@lists.linux.dev, linux-kernel@vger.kernel.org, Ajay Garg , Paolo Bonzini , Nathan Chancellor , Nick Desaulniers Subject: Re: [PATCH] KVM: x86: Shove vp_bitmap handling down into sparse_set_to_vcpu_mask() In-Reply-To: <20211028213408.2883933-1-seanjc@google.com> References: <20211028213408.2883933-1-seanjc@google.com> Date: Fri, 29 Oct 2021 14:56:05 +0200 Message-ID: <87pmrokn16.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org Sean Christopherson writes: > Move the vp_bitmap "allocation" that's need to handle mismatched vp_index > values down into sparse_set_to_vcpu_mask() and drop __always_inline from > said helper. The vp_bitmap mess is a detail that's specific to the sparse > translation and does not need to be exposed to the caller. > > The underlying motivation is to fudge around a compilation warning/error > when CONFIG_KASAN_STACK=y, which is selected (and can't be unselected) by > CONFIG_KASAN=y when compiling with gcc (clang/LLVM is a stack hog in some > cases so it's opt-in for clang). KASAN_STACK adds a redzone around every > stack variable, which pushes the Hyper-V functions over the default limit > of 1024. With CONFIG_KVM_WERROR=y, this breaks the build. Shuffling which > function is charged with vp_bitmap gets all functions below the default > limit. > > Regarding the __always_inline, prior to commit f21dd494506a ("KVM: x86: > hyperv: optimize sparse VP set processing") the helper, then named > hv_vcpu_in_sparse_set(), was a tiny bit of code that effectively boiled > down to a handful of bit ops. The __always_inline was understandable, if > not justifiable. Since the aforementioned change, sparse_set_to_vcpu_mask() > is a chunky 350-450+ bytes of code without KASAN=y, and balloons to 1100+ > with KASAN=y. In other words, it has no business being forcefully inlined. > > Reported-by: Ajay Garg > Signed-off-by: Sean Christopherson > --- > > Vitaly (and anyone with extensive KVM + Hyper-V knowledge), it would be > really helpful to get better coverage in kvm-unit-tests. I can't agree more. This *is* in my backlog but unfortunately I can't give any forcast on when I'll get to it :-( > There's a smoke > test for this in selftests, but it's not really all that interesting. It > took me over an hour and a half just to get a Linux guest to hit the > relevant flows. Most of that was due to QEMU 5.1 bugs (doesn't advertise > HYPERCALL MSR by default) This should be fixed already, right? > and Linux guest stupidity (silently disables > itself if said MSR isn't available), but it was really annoying to have to > go digging through QEMU to figure out how to even enable features that are > extensive/critical enough to warrant their own tests. > > /wave to the clang folks for the pattern patch on the changelog ;-) > > arch/x86/kvm/hyperv.c | 55 ++++++++++++++++++++++--------------------- > 1 file changed, 28 insertions(+), 27 deletions(-) > > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c > index 4f15c0165c05..80018cfab5c7 100644 > --- a/arch/x86/kvm/hyperv.c > +++ b/arch/x86/kvm/hyperv.c > @@ -1710,31 +1710,36 @@ int kvm_hv_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata, bool host) > return kvm_hv_get_msr(vcpu, msr, pdata, host); > } > > -static __always_inline unsigned long *sparse_set_to_vcpu_mask( > - struct kvm *kvm, u64 *sparse_banks, u64 valid_bank_mask, > - u64 *vp_bitmap, unsigned long *vcpu_bitmap) > +static void sparse_set_to_vcpu_mask(struct kvm *kvm, u64 *sparse_banks, > + u64 valid_bank_mask, unsigned long *vcpu_mask) > { > struct kvm_hv *hv = to_kvm_hv(kvm); > + bool has_mismatch = atomic_read(&hv->num_mismatched_vp_indexes); > + u64 vp_bitmap[KVM_HV_MAX_SPARSE_VCPU_SET_BITS]; > struct kvm_vcpu *vcpu; > int i, bank, sbank = 0; > + u64 *bitmap; > > - memset(vp_bitmap, 0, > - KVM_HV_MAX_SPARSE_VCPU_SET_BITS * sizeof(*vp_bitmap)); > + BUILD_BUG_ON(sizeof(vp_bitmap) > > + sizeof(*vcpu_mask) * BITS_TO_LONGS(KVM_MAX_VCPUS)); > + > + /* If vp_index == vcpu_idx for all vCPUs, fill vcpu_mask directly. */ > + if (likely(!has_mismatch)) > + bitmap = (u64 *)vcpu_mask; > + > + memset(bitmap, 0, sizeof(vp_bitmap)); ... but in the unlikely case has_mismatch == true 'bitmap' is still uninitialized here, right? How doesn't it crash? > for_each_set_bit(bank, (unsigned long *)&valid_bank_mask, > KVM_HV_MAX_SPARSE_VCPU_SET_BITS) > - vp_bitmap[bank] = sparse_banks[sbank++]; > + bitmap[bank] = sparse_banks[sbank++]; > > - if (likely(!atomic_read(&hv->num_mismatched_vp_indexes))) { > - /* for all vcpus vp_index == vcpu_idx */ > - return (unsigned long *)vp_bitmap; > - } > + if (likely(!has_mismatch)) > + return; > > - bitmap_zero(vcpu_bitmap, KVM_MAX_VCPUS); > + bitmap_zero(vcpu_mask, KVM_MAX_VCPUS); > kvm_for_each_vcpu(i, vcpu, kvm) { > if (test_bit(kvm_hv_get_vpindex(vcpu), (unsigned long *)vp_bitmap)) 'vp_bitmap' also doesn't seem to be assigned to anything, I'm really confused :-( Didn't you accidentally mix up 'vp_bitmap' and 'bitmap'? > - __set_bit(i, vcpu_bitmap); > + __set_bit(i, vcpu_mask); > } > - return vcpu_bitmap; > } > > struct kvm_hv_hcall { > @@ -1756,9 +1761,7 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool > struct kvm *kvm = vcpu->kvm; > struct hv_tlb_flush_ex flush_ex; > struct hv_tlb_flush flush; > - u64 vp_bitmap[KVM_HV_MAX_SPARSE_VCPU_SET_BITS]; > - DECLARE_BITMAP(vcpu_bitmap, KVM_MAX_VCPUS); > - unsigned long *vcpu_mask; > + DECLARE_BITMAP(vcpu_mask, KVM_MAX_VCPUS); > u64 valid_bank_mask; > u64 sparse_banks[64]; > int sparse_banks_len; > @@ -1842,11 +1845,9 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool > if (all_cpus) { > kvm_make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH_GUEST); > } else { > - vcpu_mask = sparse_set_to_vcpu_mask(kvm, sparse_banks, valid_bank_mask, > - vp_bitmap, vcpu_bitmap); > + sparse_set_to_vcpu_mask(kvm, sparse_banks, valid_bank_mask, vcpu_mask); > > - kvm_make_vcpus_request_mask(kvm, KVM_REQ_TLB_FLUSH_GUEST, > - vcpu_mask); > + kvm_make_vcpus_request_mask(kvm, KVM_REQ_TLB_FLUSH_GUEST, vcpu_mask); We're not bound by 80-char limit anymore, are we? :-) > } > > ret_success: > @@ -1879,9 +1880,7 @@ static u64 kvm_hv_send_ipi(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool > struct kvm *kvm = vcpu->kvm; > struct hv_send_ipi_ex send_ipi_ex; > struct hv_send_ipi send_ipi; > - u64 vp_bitmap[KVM_HV_MAX_SPARSE_VCPU_SET_BITS]; > - DECLARE_BITMAP(vcpu_bitmap, KVM_MAX_VCPUS); > - unsigned long *vcpu_mask; > + DECLARE_BITMAP(vcpu_mask, KVM_MAX_VCPUS); > unsigned long valid_bank_mask; > u64 sparse_banks[64]; > int sparse_banks_len; > @@ -1937,11 +1936,13 @@ static u64 kvm_hv_send_ipi(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool > if ((vector < HV_IPI_LOW_VECTOR) || (vector > HV_IPI_HIGH_VECTOR)) > return HV_STATUS_INVALID_HYPERCALL_INPUT; > > - vcpu_mask = all_cpus ? NULL : > - sparse_set_to_vcpu_mask(kvm, sparse_banks, valid_bank_mask, > - vp_bitmap, vcpu_bitmap); > + if (all_cpus) { > + kvm_send_ipi_to_many(kvm, vector, NULL); > + } else { > + sparse_set_to_vcpu_mask(kvm, sparse_banks, valid_bank_mask, vcpu_mask); > > - kvm_send_ipi_to_many(kvm, vector, vcpu_mask); > + kvm_send_ipi_to_many(kvm, vector, vcpu_mask); > + } > > ret_success: > return HV_STATUS_SUCCESS; -- Vitaly