From: Zenghui Yu <yuzenghui@xxxxxxxxxx> stage2_unmap_vm() was introduced to unmap user RAM region in the stage2 page table to make the caches coherent. E.g., a guest reboot with stage1 MMU disabled will access memory using non-cacheable attributes. If the RAM and caches are not coherent at this stage, some evicted dirty cache line may go and corrupt guest data in RAM. Since ARMv8.4, S2FWB feature is mandatory and KVM will take advantage of it to configure the stage2 page table and the attributes of memory access. So we ensure that guests always access memory using cacheable attributes and thus, the caches always be coherent. So on CPUs that support S2FWB, we can safely reset the vcpu without a heavy stage2 unmapping. Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx> Link: https://lore.kernel.org/r/20200415072835.1164-1-yuzenghui@xxxxxxxxxx --- arch/arm64/kvm/arm.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c index ee1b5bba1d08..0ea9a0266d9a 100644 --- a/arch/arm64/kvm/arm.c +++ b/arch/arm64/kvm/arm.c @@ -983,8 +983,11 @@ static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu, /* * Ensure a rebooted VM will fault in RAM pages and detect if the * guest MMU is turned off and flush the caches as needed. + * + * S2FWB enforces all memory accesses to RAM being cacheable, we + * ensure that the cache is always coherent. */ - if (vcpu->arch.has_run_once) + if (vcpu->arch.has_run_once && !cpus_have_const_cap(ARM64_HAS_STAGE2_FWB)) stage2_unmap_vm(vcpu->kvm); vcpu_reset_hcr(vcpu); -- 2.26.2
