Re: [PATCH LINUX RFC] xen: privcmd: implement IOCTL_PRIVCMD_MMAPBATCH_V2_UNCACHED

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, Dec 18, 2013 at 05:30:37PM +0000, Ian Campbell wrote:
> On ARM we want to use uncached foreign mappings while building the domain
> because the guests start with MMU and caches disabled.
> 

Why introduce a new ioctl? Could we piggyback on the old one and on ARM
do the uncached.

> Flushing the caches before launching the guest is problematic because there is
> a window between flush and unmap where the processor might speculatively fill
> a cache line. Using a non-cacheable mapping completely avoids this.
> 
> Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
> Cc: Stefano.Stabellini@citrix.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: David Vrabel <david.vrabel@citrix.com>
> Cc: xen-devel@lists.xenproject.org
> ---
>  drivers/xen/privcmd.c      |   15 ++++++++++-----
>  include/uapi/xen/privcmd.h |    2 ++
>  2 files changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> index 569a13b..b5561d1 100644
> --- a/drivers/xen/privcmd.c
> +++ b/drivers/xen/privcmd.c
> @@ -253,6 +253,7 @@ struct mmap_batch_state {
>  	domid_t domain;
>  	unsigned long va;
>  	struct vm_area_struct *vma;
> +	pgprot_t prot;
>  	int index;
>  	/* A tristate:
>  	 *      0 for no errors
> @@ -285,8 +286,7 @@ static int mmap_batch_fn(void *data, void *state)
>  		cur_page = pages[st->index++];
>  
>  	ret = xen_remap_domain_mfn_range(st->vma, st->va & PAGE_MASK, *mfnp, 1,
> -					 st->vma->vm_page_prot, st->domain,
> -					 &cur_page);
> +					 st->prot, st->domain, &cur_page);
>  
>  	/* Store error code for second pass. */
>  	if (st->version == 1) {
> @@ -367,7 +367,7 @@ static int alloc_empty_pages(struct vm_area_struct *vma, int numpgs)
>  
>  static struct vm_operations_struct privcmd_vm_ops;
>  
> -static long privcmd_ioctl_mmap_batch(void __user *udata, int version)
> +static long privcmd_ioctl_mmap_batch(void __user *udata, int version, int cached)
>  {
>  	int ret;
>  	struct privcmd_mmapbatch_v2 m;
> @@ -464,6 +464,8 @@ static long privcmd_ioctl_mmap_batch(void __user *udata, int version)
>  
>  	state.domain        = m.dom;
>  	state.vma           = vma;
> +	state.prot          = cached ? vma->vm_page_prot
> +				     : pgprot_noncached(vma->vm_page_prot);
>  	state.va            = m.addr;
>  	state.index         = 0;
>  	state.global_error  = 0;
> @@ -514,13 +516,16 @@ static long privcmd_ioctl(struct file *file,
>  		break;
>  
>  	case IOCTL_PRIVCMD_MMAPBATCH:
> -		ret = privcmd_ioctl_mmap_batch(udata, 1);
> +		ret = privcmd_ioctl_mmap_batch(udata, 1, 1);
>  		break;
>  
>  	case IOCTL_PRIVCMD_MMAPBATCH_V2:
> -		ret = privcmd_ioctl_mmap_batch(udata, 2);
> +		ret = privcmd_ioctl_mmap_batch(udata, 2, 1);
>  		break;
>  
> +	case IOCTL_PRIVCMD_MMAPBATCH_V2_UNCACHED:
> +		ret = privcmd_ioctl_mmap_batch(udata, 2, 0);
> +		break;
>  	default:
>  		ret = -EINVAL;
>  		break;
> diff --git a/include/uapi/xen/privcmd.h b/include/uapi/xen/privcmd.h
> index a853168..be7e72b 100644
> --- a/include/uapi/xen/privcmd.h
> +++ b/include/uapi/xen/privcmd.h
> @@ -94,5 +94,7 @@ struct privcmd_mmapbatch_v2 {
>  	_IOC(_IOC_NONE, 'P', 3, sizeof(struct privcmd_mmapbatch))
>  #define IOCTL_PRIVCMD_MMAPBATCH_V2				\
>  	_IOC(_IOC_NONE, 'P', 4, sizeof(struct privcmd_mmapbatch_v2))
> +#define IOCTL_PRIVCMD_MMAPBATCH_V2_UNCACHED			\
> +	_IOC(_IOC_NONE, 'P', 5, sizeof(struct privcmd_mmapbatch_v2))
>  
>  #endif /* __LINUX_PUBLIC_PRIVCMD_H__ */
> -- 
> 1.7.10.4
>