Re: KVM: PCIPT: direct mmio

[Avi Kivity <avi@qumranet.com>]

Ben-Ami Yassour wrote:
> Amit,
>
> Below is the patch for PCI passthrough tree, it enables a guest to 
> access a device's
> memory mapped I/O regions directly, without requiring the host to trap 
> and
> emulate every MMIO access.
>
> This patch requires only userspace changes and it is relaying on the 
> kernel patch by Anthony: "Handle vma regions with no backing page". 
> Note that this patch requires CONFIG_NUMA to be set. It does require a 
> change to the VT-d that Allen sent a while ago, to avoid mapping of 
> memory slots with no backing page.
>
>
> diff --git a/libkvm/libkvm.c b/libkvm/libkvm.c
> index d1e95a4..ce062cb 100644
> --- a/libkvm/libkvm.c
> +++ b/libkvm/libkvm.c
> @@ -400,7 +400,7 @@ void *kvm_create_userspace_phys_mem(kvm_context_t 
> kvm, unsigned long phys_start,
>  {
>      int r;
>      int prot = PROT_READ;
> -    void *ptr;
> +    void *ptr = NULL;
>      struct kvm_userspace_memory_region memory = {
>          .memory_size = len,
>          .guest_phys_addr = phys_start,
> @@ -410,16 +410,24 @@ void 
> *kvm_create_userspace_phys_mem(kvm_context_t kvm, unsigned long 
> phys_start,
>      if (writable)
>          prot |= PROT_WRITE;
>
> -    ptr = mmap(NULL, len, prot, MAP_ANONYMOUS | MAP_SHARED, -1, 0);
> -    if (ptr == MAP_FAILED) {
> -        fprintf(stderr, "create_userspace_phys_mem: %s", 
> strerror(errno));
> -        return 0;
> -    }
> +    if (len > 0) {
> +        ptr = mmap(NULL, len, prot, MAP_ANONYMOUS | MAP_SHARED, -1, 0);
> +        if (ptr == MAP_FAILED) {
> +            fprintf(stderr, "create_userspace_phys_mem: %s",
> +                strerror(errno));
> +            return 0;
> +        }

You're using 'len == 0' here to change the semantics of the function.  
It would be better to have two different APIs (perhaps sharing some of 
the implementation by calling a helper).

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.