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[34.76.60.220]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47f63ee926dsm35363290f8f.35.2026.07.21.03.19.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 21 Jul 2026 03:19:25 -0700 (PDT) Date: Tue, 21 Jul 2026 10:19:22 +0000 From: Mostafa Saleh To: Marc Zyngier Cc: linux-kernel@vger.kernel.org, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, oupton@kernel.org, seiden@linux.ibm.com, joey.gouly@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, catalin.marinas@arm.com, will@kernel.org, vdonnefort@google.com, tabba@google.com, sebastianene@google.com, keirf@google.com, Fuad Tabba Subject: Re: [PATCH v2] KVM: arm64: Optimize protected mode and FWB Message-ID: References: <20260721085646.1660673-1-smostafa@google.com> <86y0f4ejr4.wl-maz@kernel.org> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <86y0f4ejr4.wl-maz@kernel.org> On Tue, Jul 21, 2026 at 11:10:07AM +0100, Marc Zyngier wrote: > On Tue, 21 Jul 2026 09:56:45 +0100, > Mostafa Saleh wrote: > > > > KVM opportunistically enables FWB if supported by the system for guest > > VMs, which allows it to elude cache maintenance for data as they are > > forced to be cacheable from stage-2. > > In that case, __clean_dcache_guest_page() will immediately return. > > However in protected mode, before calling __clean_dcache_guest_page() > > it loops over the range and fix_map/unmap it, issuing TLB > > invalidations, dsb() and isb() unnecessarily. > > > > This can be optimized by returning early if FWB is supported, > > kvm_pgtable_stage2_map() already issues dsb() and tlb invalidation > > functions issue dsb() for the unmap path. > > > > Signed-off-by: Mostafa Saleh > > Reviewed-by: Fuad Tabba > > --- > > arch/arm64/kvm/hyp/nvhe/mem_protect.c | 7 +++++++ > > 1 file changed, 7 insertions(+) > > > > diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c > > index 4e329e39a695..049a09be2bbf 100644 > > --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c > > +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c > > @@ -261,6 +261,13 @@ static void __apply_guest_page(void *va, size_t size, > > > > static void clean_dcache_guest_page(void *va, size_t size) > > { > > + /* > > + * Guest stage-2 uses FWB if available, making it safe to elide CMOs. > > + * In contrast, the host stage-2 never has FWB enabled. > > Beeeeeeep!!! Since a373930ec9406 ("KVM: arm64: Switch pKVM host S2 > over to KVM_PGTABLE_S2_AS_S1"), FWB is enabled everywhere when > available. > > Not sure that changes anything, but the comment is factually wrong. Ah, I see it uses KVM_PGTABLE_S2_AS_S1 in that case, that should not change the patch, I will need to update the comment. Thanks, Mostafa > > Thanks, > > M. > > -- > Without deviation from the norm, progress is not possible.