Re: [PATCH 04/10] KVM: Introduce a new guest mapping API

Paolo Bonzini
2018-04-12T14:33:15Z

Coming back to this (nice) series.

On 21/02/2018 18:47, KarimAllah Ahmed wrote:
> +bool kvm_vcpu_map(struct kvm_vcpu *vcpu, gfn_t gfn, struct kvm_host_map *map)
> +{
> + kvm_pfn_t pfn;
> + void *kaddr = NULL;

Can we s/kaddr/hva/ since that's the standard nomenclature?

> + struct page *page = NULL;
> +
> + if (map->kaddr && map->gfn == gfn)
> + /* If the mapping is valid and guest memory is already mapped */
> + return true;

Please return 0/-EINVAL instead. More important, this optimization is
problematic because:

1) the underlying memslots array could have changed. You'd also need to
store the generation count (see kvm_read_guest_cached for an example)

2) worse, the memslots array could have switched between the SMM and
non-SMM address spaces. This is by the way the reason why there is no
kvm_vcpu_read_guest_cached API.

However, all the places you're changing in patches 4-10 are doing
already kvm_vcpu_gpa_to_page, so I suggest just dropping this optimization.

> + else if (map->kaddr)
> + /* If the mapping is valid but trying to map a different guest pfn */
> + kvm_vcpu_unmap(map);
> +
> + pfn = kvm_vcpu_gfn_to_pfn(vcpu, gfn);

Please change the API to:

static int __kvm_map_gfn(struct kvm_memslot *memslots, gfn_t gfn,
struct kvm_host_map *map)

calling gfn_to_pfn_memslot(memslots, gfn)

int kvm_vcpu_map_gfn(struct kvm_vcpu *vcpu gfn_t gfn,
struct kvm_host_map *map)

calling kvm_vcpu_gfn_to_memslot + __kvm_map

void kvm_unmap_gfn(struct kvm_host_map *map)

> + if (is_error_pfn(pfn))
> + return false;

Should this be is_error_noslot_pfn?

> + if (pfn_valid(pfn)) {
> + page = pfn_to_page(pfn);
> + kaddr = vmap(&page, 1, VM_MAP, PAGE_KERNEL);
> + } else {
> + kaddr = memremap(pfn_to_hpa(pfn), PAGE_SIZE, MEMREMAP_WB);
> + }
> +
> + if (!kaddr)
> + return false;
> +
> + map->page = page;
> + map->kaddr = kaddr;
> + map->pfn = pfn;
> + map->gfn = gfn;
> +
> + return true;
> +}

>
> +void kvm_vcpu_unmap(struct kvm_host_map *map)
> +{
> + if (!map->kaddr)
> + return;
> +
> + if (map->page)
> + kunmap(map->page);
> + else
> + memunmap(map->kaddr);
> +
> + kvm_release_pfn_dirty(map->pfn);
> + memset(map, 0, sizeof(*map));

This can clear just map->kaddr (map->hva after the above review).

Thanks,

Paolo

> +}
> +