Re: [RFC PATCH 2/8] KVM: Add KVM_MAP_MEMORY vcpu ioctl to pre-populate guest memory

Sean Christopherson
2024-03-11T17:23:30Z

On Fri, Mar 01, 2024, isaku.yamahata@intel.com wrote:
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d1fd9cb5d037..d77c9b79d76b 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4419,6 +4419,69 @@ static int kvm_vcpu_ioctl_get_stats_fd(struct kvm_vcpu *vcpu)
> return fd;
> }
>
> +__weak int kvm_arch_vcpu_pre_map_memory(struct kvm_vcpu *vcpu)
> +{
> + return -EOPNOTSUPP;
> +}
> +
> +__weak int kvm_arch_vcpu_map_memory(struct kvm_vcpu *vcpu,
> + struct kvm_memory_mapping *mapping)
> +{
> + return -EOPNOTSUPP;
> +}
> +
> +static int kvm_vcpu_map_memory(struct kvm_vcpu *vcpu,
> + struct kvm_memory_mapping *mapping)
> +{
> + bool added = false;
> + int idx, r = 0;

Pointless initialization of 'r'.

> +
> + if (mapping->flags & ~(KVM_MEMORY_MAPPING_FLAG_WRITE |
> + KVM_MEMORY_MAPPING_FLAG_EXEC |
> + KVM_MEMORY_MAPPING_FLAG_USER |
> + KVM_MEMORY_MAPPING_FLAG_PRIVATE))
> + return -EINVAL;
> + if ((mapping->flags & KVM_MEMORY_MAPPING_FLAG_PRIVATE) &&
> + !kvm_arch_has_private_mem(vcpu->kvm))
> + return -EINVAL;
> +
> + /* Sanity check */

Pointless comment.

> + if (!IS_ALIGNED(mapping->source, PAGE_SIZE) ||
> + !mapping->nr_pages ||

> + mapping->base_gfn + mapping->nr_pages <= mapping->base_gfn)
> + return -EINVAL;
> +
> + vcpu_load(vcpu);
> + idx = srcu_read_lock(&vcpu->kvm->srcu);
> + r = kvm_arch_vcpu_pre_map_memory(vcpu);

This hooks is unnecessary, x86's kvm_mmu_reload() is optimized for the happy path
where the MMU is already loaded. Just make the call from kvm_arch_vcpu_map_memory().

> + if (r)
> + return r;

Which is a good thing, because this leaks the SRCU lock.

> +
> + while (mapping->nr_pages) {
> + if (signal_pending(current)) {
> + r = -ERESTARTSYS;

Why -ERESTARTSYS instead of -EINTR? The latter is KVM's typical response to a
pending signal.

> + break;
> + }
> +
> + if (need_resched())

No need to manually check need_resched(), the below is a _conditional_ resched.
The reason KVM explicitly checks need_resched() in MMU flows is because KVM needs
to drop mmu_lock before rescheduling, i.e. calling cond_resched() directly would
try to schedule() while holding a spinlock.

> + cond_resched();
> +
> + r = kvm_arch_vcpu_map_memory(vcpu, mapping);
> + if (r)
> + break;
> +
> + added = true;
> + }
> +
> + srcu_read_unlock(&vcpu->kvm->srcu, idx);
> + vcpu_put(vcpu);
> +
> + if (added && mapping->nr_pages > 0)
> + r = -EAGAIN;

No, this clobbers 'r', which might hold a fatal error code. I don't see any
reason for common code to ever force -EAGAIN, it can't possibly know if trying
again is reasonable.

> +
> + return r;
> +}