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Fri, 29 Aug 2025 13:47:26 -0700 (PDT) Date: Fri, 29 Aug 2025 13:47:25 -0700 In-Reply-To: <20250829-pmu_event_info-v5-6-9dca26139a33@rivosinc.com> Mime-Version: 1.0 References: <20250829-pmu_event_info-v5-0-9dca26139a33@rivosinc.com> <20250829-pmu_event_info-v5-6-9dca26139a33@rivosinc.com> Message-ID: Subject: Re: [PATCH v5 6/9] KVM: Add a helper function to check if a gpa is in writable memselot From: Sean Christopherson To: Atish Patra Cc: Anup Patel , Will Deacon , Mark Rutland , Paul Walmsley , Palmer Dabbelt , Mayuresh Chitale , linux-riscv@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, kvm-riscv@lists.infradead.org X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250829_134727_960082_F8BF749C X-CRM114-Status: GOOD ( 22.31 ) X-BeenThere: kvm-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "kvm-riscv" Errors-To: kvm-riscv-bounces+kvm-riscv=archiver.kernel.org@lists.infradead.org On Fri, Aug 29, 2025, Atish Patra wrote: > The arch specific code may need to know if a particular gpa is valid and > writable for the shared memory between the host and the guest. Currently, > there are few places where it is used in RISC-V implementation. Given the > nature of the function it may be used for other architectures. > Hence, a common helper function is added. > > Suggested-by: Sean Christopherson > Signed-off-by: Atish Patra > --- > include/linux/kvm_host.h | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index 15656b7fba6c..eec5cbbcb4b3 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -1892,6 +1892,14 @@ static inline bool kvm_is_gpa_in_memslot(struct kvm *kvm, gpa_t gpa) > return !kvm_is_error_hva(hva); > } > > +static inline bool kvm_is_gpa_in_writable_memslot(struct kvm *kvm, gpa_t gpa) > +{ > + bool writable; > + unsigned long hva = gfn_to_hva_prot(kvm, gpa_to_gfn(gpa), &writable); > + > + return !kvm_is_error_hva(hva) && writable; I don't hate this API, but I don't love it either. Because knowing that the _memslot_ is writable doesn't mean all that much. E.g. in this usage: hva = kvm_vcpu_gfn_to_hva_prot(vcpu, shmem >> PAGE_SHIFT, &writable); if (kvm_is_error_hva(hva) || !writable) return SBI_ERR_INVALID_ADDRESS; ret = kvm_vcpu_write_guest(vcpu, shmem, &zero_sta, sizeof(zero_sta)); if (ret) return SBI_ERR_FAILURE; the error code returned to the guest will be different if the memslot is read-only versus if the VMA is read-only (or not even mapped!). Unless every read-only memslot is explicitly communicated as such to the guest, I don't see how the guest can *know* that a memslot is read-only, so returning INVALID_ADDRESS in that case but not when the underlying VMA isn't writable seems odd. It's also entirely possible the memslot could be replaced with a read-only memslot after the check, or vice versa, i.e. become writable after being rejected. Is it *really* a problem to return FAILURE if the guest attempts to setup steal-time in a read-only memslot? I.e. why not do this and call it good? if (!kvm_is_gpa_in_memslot(vcpu->kvm, shmem >> PAGE_SHIFT)) return SBI_ERR_INVALID_ADDRESS; ret = kvm_vcpu_write_guest(vcpu, shmem, &zero_sta, sizeof(zero_sta)); if (ret) return SBI_ERR_FAILURE; -- kvm-riscv mailing list kvm-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/kvm-riscv From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f74.google.com (mail-pj1-f74.google.com [209.85.216.74]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 98FBA19D07A for ; Fri, 29 Aug 2025 20:47:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.74 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756500449; cv=none; b=kuho8r0JKytQi4CQjUKzQwsz0NCDN5cAzBnKZO3eDhgPxODp82hB7fWrcBIURZUb22lRbHpWMdiB182W3Vwjf3+eS2yEUHse/sVGx5TdbAx9nCxt6CnLkI4rUcDie3Gg3wlWj6meGgLAKVUcKApXZNg8c9vYvTL8rbCOzS304qk= ARC-Message-Signature:i=1; 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charset="us-ascii" On Fri, Aug 29, 2025, Atish Patra wrote: > The arch specific code may need to know if a particular gpa is valid and > writable for the shared memory between the host and the guest. Currently, > there are few places where it is used in RISC-V implementation. Given the > nature of the function it may be used for other architectures. > Hence, a common helper function is added. > > Suggested-by: Sean Christopherson > Signed-off-by: Atish Patra > --- > include/linux/kvm_host.h | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index 15656b7fba6c..eec5cbbcb4b3 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -1892,6 +1892,14 @@ static inline bool kvm_is_gpa_in_memslot(struct kvm *kvm, gpa_t gpa) > return !kvm_is_error_hva(hva); > } > > +static inline bool kvm_is_gpa_in_writable_memslot(struct kvm *kvm, gpa_t gpa) > +{ > + bool writable; > + unsigned long hva = gfn_to_hva_prot(kvm, gpa_to_gfn(gpa), &writable); > + > + return !kvm_is_error_hva(hva) && writable; I don't hate this API, but I don't love it either. Because knowing that the _memslot_ is writable doesn't mean all that much. E.g. in this usage: hva = kvm_vcpu_gfn_to_hva_prot(vcpu, shmem >> PAGE_SHIFT, &writable); if (kvm_is_error_hva(hva) || !writable) return SBI_ERR_INVALID_ADDRESS; ret = kvm_vcpu_write_guest(vcpu, shmem, &zero_sta, sizeof(zero_sta)); if (ret) return SBI_ERR_FAILURE; the error code returned to the guest will be different if the memslot is read-only versus if the VMA is read-only (or not even mapped!). Unless every read-only memslot is explicitly communicated as such to the guest, I don't see how the guest can *know* that a memslot is read-only, so returning INVALID_ADDRESS in that case but not when the underlying VMA isn't writable seems odd. It's also entirely possible the memslot could be replaced with a read-only memslot after the check, or vice versa, i.e. become writable after being rejected. Is it *really* a problem to return FAILURE if the guest attempts to setup steal-time in a read-only memslot? 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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org On Fri, Aug 29, 2025, Atish Patra wrote: > The arch specific code may need to know if a particular gpa is valid and > writable for the shared memory between the host and the guest. Currently, > there are few places where it is used in RISC-V implementation. Given the > nature of the function it may be used for other architectures. > Hence, a common helper function is added. > > Suggested-by: Sean Christopherson > Signed-off-by: Atish Patra > --- > include/linux/kvm_host.h | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index 15656b7fba6c..eec5cbbcb4b3 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -1892,6 +1892,14 @@ static inline bool kvm_is_gpa_in_memslot(struct kvm *kvm, gpa_t gpa) > return !kvm_is_error_hva(hva); > } > > +static inline bool kvm_is_gpa_in_writable_memslot(struct kvm *kvm, gpa_t gpa) > +{ > + bool writable; > + unsigned long hva = gfn_to_hva_prot(kvm, gpa_to_gfn(gpa), &writable); > + > + return !kvm_is_error_hva(hva) && writable; I don't hate this API, but I don't love it either. Because knowing that the _memslot_ is writable doesn't mean all that much. E.g. in this usage: hva = kvm_vcpu_gfn_to_hva_prot(vcpu, shmem >> PAGE_SHIFT, &writable); if (kvm_is_error_hva(hva) || !writable) return SBI_ERR_INVALID_ADDRESS; ret = kvm_vcpu_write_guest(vcpu, shmem, &zero_sta, sizeof(zero_sta)); if (ret) return SBI_ERR_FAILURE; the error code returned to the guest will be different if the memslot is read-only versus if the VMA is read-only (or not even mapped!). Unless every read-only memslot is explicitly communicated as such to the guest, I don't see how the guest can *know* that a memslot is read-only, so returning INVALID_ADDRESS in that case but not when the underlying VMA isn't writable seems odd. It's also entirely possible the memslot could be replaced with a read-only memslot after the check, or vice versa, i.e. become writable after being rejected. Is it *really* a problem to return FAILURE if the guest attempts to setup steal-time in a read-only memslot? I.e. why not do this and call it good? if (!kvm_is_gpa_in_memslot(vcpu->kvm, shmem >> PAGE_SHIFT)) return SBI_ERR_INVALID_ADDRESS; ret = kvm_vcpu_write_guest(vcpu, shmem, &zero_sta, sizeof(zero_sta)); if (ret) return SBI_ERR_FAILURE; _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv