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King (Oracle)" To: Yury Norov Cc: Marc Zyngier , Luo Jie , Rasmus Villemoes , Julia Lawall , Nicolas Palix , Catalin Marinas , Will Deacon , Oliver Upton , Joey Gouly , Suzuki K Poulose , Zenghui Yu , linux-kernel@vger.kernel.org, cocci@inria.fr, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, andrew@lunn.ch, quic_kkumarcs@quicinc.com, quic_linchen@quicinc.com, quic_leiwei@quicinc.com, quic_suruchia@quicinc.com, quic_pavir@quicinc.com Message-ID: References: <20250417-field_modify-v3-0-6f7992aafcb7@quicinc.com> <20250417-field_modify-v3-4-6f7992aafcb7@quicinc.com> <86r01rjald.wl-maz@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: cocci-request@inria.fr X-Validation-by: victor.gambier@inria.fr Subject: Re: [cocci] [PATCH v3 4/6] arm64: nvhe: Convert the opencoded field modify Reply-To: "Russell King (Oracle)" X-Loop: cocci@inria.fr X-Sequence: 2718 Errors-To: cocci-owner@inria.fr Precedence: list Precedence: bulk X-no-archive: yes List-Id: List-Help: List-Subscribe: List-Unsubscribe: List-Post: List-Owner: List-Archive: Archived-At: On Wed, Apr 23, 2025 at 02:27:06PM -0400, Yury Norov wrote: > On Wed, Apr 23, 2025 at 06:48:34PM +0100, Russell King (Oracle) wrote: > > On Fri, Apr 18, 2025 at 11:14:48AM -0400, Yury Norov wrote: > > > On Thu, Apr 17, 2025 at 12:23:10PM +0100, Marc Zyngier wrote: > > > > On Thu, 17 Apr 2025 11:47:11 +0100, > > > > Luo Jie wrote: > > > > > > > > > > Replaced below code with the wrapper FIELD_MODIFY(MASK, ®, val) > > > > > - reg &= ~MASK; > > > > > - reg |= FIELD_PREP(MASK, val); > > > > > The semantic patch that makes this change is available > > > > > in scripts/coccinelle/misc/field_modify.cocci. > > > > > > > > > > More information about semantic patching is available at > > > > > https://coccinelle.gitlabpages.inria.fr/website > > > > > > > > > > Signed-off-by: Luo Jie > > > > > --- > > > > > arch/arm64/kvm/hyp/include/nvhe/memory.h | 3 +-- > > > > > 1 file changed, 1 insertion(+), 2 deletions(-) > > > > > > > > > > diff --git a/arch/arm64/kvm/hyp/include/nvhe/memory.h b/arch/arm64/kvm/hyp/include/nvhe/memory.h > > > > > index 34233d586060..b2af748964d0 100644 > > > > > --- a/arch/arm64/kvm/hyp/include/nvhe/memory.h > > > > > +++ b/arch/arm64/kvm/hyp/include/nvhe/memory.h > > > > > @@ -30,8 +30,7 @@ enum pkvm_page_state { > > > > > static inline enum kvm_pgtable_prot pkvm_mkstate(enum kvm_pgtable_prot prot, > > > > > enum pkvm_page_state state) > > > > > { > > > > > - prot &= ~PKVM_PAGE_STATE_PROT_MASK; > > > > > - prot |= FIELD_PREP(PKVM_PAGE_STATE_PROT_MASK, state); > > > > > + FIELD_MODIFY(PKVM_PAGE_STATE_PROT_MASK, &prot, state); > > > > > return prot; > > > > > } > > > > > > > > Following up on my suggestion to *not* add anything new, this patch > > > > could be written as: > > > > > > > > diff --git a/arch/arm64/kvm/hyp/include/nvhe/memory.h b/arch/arm64/kvm/hyp/include/nvhe/memory.h > > > > index 34233d5860607..08cb6ba0e0716 100644 > > > > --- a/arch/arm64/kvm/hyp/include/nvhe/memory.h > > > > +++ b/arch/arm64/kvm/hyp/include/nvhe/memory.h > > > > @@ -30,9 +30,8 @@ enum pkvm_page_state { > > > > static inline enum kvm_pgtable_prot pkvm_mkstate(enum kvm_pgtable_prot prot, > > > > enum pkvm_page_state state) > > > > { > > > > - prot &= ~PKVM_PAGE_STATE_PROT_MASK; > > > > - prot |= FIELD_PREP(PKVM_PAGE_STATE_PROT_MASK, state); > > > > - return prot; > > > > + u64 p = prot; > > > > + return u64_replace_bits(p, state, PKVM_PAGE_STATE_PROT_MASK); > > > > } > > > > > > This is a great example where u64_replace_bit() should NOT be used. > > > > Why not? Explain it. Don't leave people in the dark, because right > > now it looks like it's purely a religous fanaticism about what > > should and should not be used. Where's the technical reasoning? > > Because enum is an integer, i.e. 32-bit type. This statement is false, in this case. The kernel currently uses -std=gnu11, and GNU tends to be more relaxed about things, and while the C standard may say that enums are ints, that isn't the case - gcc appears to follow C++ and allow enums that are wider than ints. $ aarch64-linux-gnu-gcc -S -o - -std=gnu99 -x c - enum foo { A = 1L << 0, B = 1L << 53, }; int main() { return sizeof(enum foo); } Gives the following code: main: .LFB0: .cfi_startproc mov w0, 8 ret .cfi_endproc meaning that sizeof(enum foo) is 8 or 64-bit. If B were 1L << 31, then sizeof(enum foo) is 4. > Now, the snippet above > typecasts it to 64-bit fixed size type, passes to 64-bit fixed-type > function, and the returned value is typecasted back to 32-bit int. In this case, the enum is defined using: KVM_PGTABLE_PROT_X = BIT(0), KVM_PGTABLE_PROT_W = BIT(1), KVM_PGTABLE_PROT_R = BIT(2), KVM_PGTABLE_PROT_DEVICE = BIT(3), KVM_PGTABLE_PROT_NORMAL_NC = BIT(4), KVM_PGTABLE_PROT_SW0 = BIT(55), KVM_PGTABLE_PROT_SW1 = BIT(56), KVM_PGTABLE_PROT_SW2 = BIT(57), KVM_PGTABLE_PROT_SW3 = BIT(58), As it contains bits beyond bit 31, and we use -std=gnu11 when building the kernel, this enum is represented using a 64-bit integer type. So, the casting to a u64 is not increasing the size of the enum, and the return value is not getting truncated down to 32-bits. > Doesn't sound the most efficient solution, right? On 32-bit arch it > may double the function size, I guess. Given that there's no inefficiency here, and that this is arm64 code which is a 64-bit arch, both those points you mention seem to be incorrect or not relevant. > But the most important is that if we adopt this practice and spread it > around, it will be really easy to overflow the 32-bit storage. The > compiler will keep silence about that. Given that in Marc's suggestion, "prot" is a 64-bit value, it's being assigned to a u64, which is then being operated on by the u64 variant of _replace_bits(), which returns the u64 result, which then gets returned as a 64-bit enum, there is no issue here as far as I can see. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last! 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charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: Russell King (Oracle) On Wed, Apr 23, 2025 at 02:27:06PM -0400, Yury Norov wrote: > On Wed, Apr 23, 2025 at 06:48:34PM +0100, Russell King (Oracle) wrote: > > On Fri, Apr 18, 2025 at 11:14:48AM -0400, Yury Norov wrote: > > > On Thu, Apr 17, 2025 at 12:23:10PM +0100, Marc Zyngier wrote: > > > > On Thu, 17 Apr 2025 11:47:11 +0100, > > > > Luo Jie wrote: > > > > > > > > > > Replaced below code with the wrapper FIELD_MODIFY(MASK, ®, val) > > > > > - reg &= ~MASK; > > > > > - reg |= FIELD_PREP(MASK, val); > > > > > The semantic patch that makes this change is available > > > > > in scripts/coccinelle/misc/field_modify.cocci. > > > > > > > > > > More information about semantic patching is available at > > > > > https://coccinelle.gitlabpages.inria.fr/website > > > > > > > > > > Signed-off-by: Luo Jie > > > > > --- > > > > > arch/arm64/kvm/hyp/include/nvhe/memory.h | 3 +-- > > > > > 1 file changed, 1 insertion(+), 2 deletions(-) > > > > > > > > > > diff --git a/arch/arm64/kvm/hyp/include/nvhe/memory.h b/arch/arm64/kvm/hyp/include/nvhe/memory.h > > > > > index 34233d586060..b2af748964d0 100644 > > > > > --- a/arch/arm64/kvm/hyp/include/nvhe/memory.h > > > > > +++ b/arch/arm64/kvm/hyp/include/nvhe/memory.h > > > > > @@ -30,8 +30,7 @@ enum pkvm_page_state { > > > > > static inline enum kvm_pgtable_prot pkvm_mkstate(enum kvm_pgtable_prot prot, > > > > > enum pkvm_page_state state) > > > > > { > > > > > - prot &= ~PKVM_PAGE_STATE_PROT_MASK; > > > > > - prot |= FIELD_PREP(PKVM_PAGE_STATE_PROT_MASK, state); > > > > > + FIELD_MODIFY(PKVM_PAGE_STATE_PROT_MASK, &prot, state); > > > > > return prot; > > > > > } > > > > > > > > Following up on my suggestion to *not* add anything new, this patch > > > > could be written as: > > > > > > > > diff --git a/arch/arm64/kvm/hyp/include/nvhe/memory.h b/arch/arm64/kvm/hyp/include/nvhe/memory.h > > > > index 34233d5860607..08cb6ba0e0716 100644 > > > > --- a/arch/arm64/kvm/hyp/include/nvhe/memory.h > > > > +++ b/arch/arm64/kvm/hyp/include/nvhe/memory.h > > > > @@ -30,9 +30,8 @@ enum pkvm_page_state { > > > > static inline enum kvm_pgtable_prot pkvm_mkstate(enum kvm_pgtable_prot prot, > > > > enum pkvm_page_state state) > > > > { > > > > - prot &= ~PKVM_PAGE_STATE_PROT_MASK; > > > > - prot |= FIELD_PREP(PKVM_PAGE_STATE_PROT_MASK, state); > > > > - return prot; > > > > + u64 p = prot; > > > > + return u64_replace_bits(p, state, PKVM_PAGE_STATE_PROT_MASK); > > > > } > > > > > > This is a great example where u64_replace_bit() should NOT be used. > > > > Why not? Explain it. Don't leave people in the dark, because right > > now it looks like it's purely a religous fanaticism about what > > should and should not be used. Where's the technical reasoning? > > Because enum is an integer, i.e. 32-bit type. This statement is false, in this case. The kernel currently uses -std=gnu11, and GNU tends to be more relaxed about things, and while the C standard may say that enums are ints, that isn't the case - gcc appears to follow C++ and allow enums that are wider than ints. $ aarch64-linux-gnu-gcc -S -o - -std=gnu99 -x c - enum foo { A = 1L << 0, B = 1L << 53, }; int main() { return sizeof(enum foo); } Gives the following code: main: .LFB0: .cfi_startproc mov w0, 8 ret .cfi_endproc meaning that sizeof(enum foo) is 8 or 64-bit. If B were 1L << 31, then sizeof(enum foo) is 4. > Now, the snippet above > typecasts it to 64-bit fixed size type, passes to 64-bit fixed-type > function, and the returned value is typecasted back to 32-bit int. In this case, the enum is defined using: KVM_PGTABLE_PROT_X = BIT(0), KVM_PGTABLE_PROT_W = BIT(1), KVM_PGTABLE_PROT_R = BIT(2), KVM_PGTABLE_PROT_DEVICE = BIT(3), KVM_PGTABLE_PROT_NORMAL_NC = BIT(4), KVM_PGTABLE_PROT_SW0 = BIT(55), KVM_PGTABLE_PROT_SW1 = BIT(56), KVM_PGTABLE_PROT_SW2 = BIT(57), KVM_PGTABLE_PROT_SW3 = BIT(58), As it contains bits beyond bit 31, and we use -std=gnu11 when building the kernel, this enum is represented using a 64-bit integer type. So, the casting to a u64 is not increasing the size of the enum, and the return value is not getting truncated down to 32-bits. > Doesn't sound the most efficient solution, right? On 32-bit arch it > may double the function size, I guess. Given that there's no inefficiency here, and that this is arm64 code which is a 64-bit arch, both those points you mention seem to be incorrect or not relevant. > But the most important is that if we adopt this practice and spread it > around, it will be really easy to overflow the 32-bit storage. The > compiler will keep silence about that. Given that in Marc's suggestion, "prot" is a 64-bit value, it's being assigned to a u64, which is then being operated on by the u64 variant of _replace_bits(), which returns the u64 result, which then gets returned as a 64-bit enum, there is no issue here as far as I can see. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!