Re: [Xen-devel] [PATCH v4 2/2] xen/m2p: use GNTTABOP_unmap_and_replace to reinstate the original mapping

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Sun, Aug 04, 2013 at 03:39:41PM +0100, Stefano Stabellini wrote:
> GNTTABOP_unmap_grant_ref unmaps a grant and replaces it with a 0
> mapping instead of reinstating the original mapping.
> Doing so separately would be racy.
> 
> To unmap a grant and reinstate the original mapping atomically we use
> GNTTABOP_unmap_and_replace.
> GNTTABOP_unmap_and_replace doesn't work with GNTMAP_contains_pte, so
> don't use it for kmaps.  GNTTABOP_unmap_and_replace zeroes the mapping
> passed in new_addr so we have to reinstate it, however that is a
> per-cpu mapping only used for balloon scratch pages, so we can be sure that
> it's not going to be accessed while the mapping is not valid.

This looks to be depend on a new structure, which is not in Linux kernel?
Are you missing a dependency patch?

Shouldn't we use some logic to figure out which hypercall to use if the
hypervisor does not support it?
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> Reviewed-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> CC: alex@xxxxxxxxxxx
> CC: dcrisan@xxxxxxxxxxxx
> ---
>  arch/x86/xen/p2m.c   |   22 +++++++++++++++-------
>  drivers/xen/gntdev.c |   11 ++---------
>  2 files changed, 17 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index 95fb2aa..0d4ec35 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -161,6 +161,7 @@
>  #include <asm/xen/page.h>
>  #include <asm/xen/hypercall.h>
>  #include <asm/xen/hypervisor.h>
> +#include <xen/balloon.h>
>  #include <xen/grant_table.h>
>  
>  #include "multicalls.h"
> @@ -967,7 +968,10 @@ int m2p_remove_override(struct page *page,
>       if (kmap_op != NULL) {
>               if (!PageHighMem(page)) {
>                       struct multicall_space mcs;
> -                     struct gnttab_unmap_grant_ref *unmap_op;
> +                     struct gnttab_unmap_and_replace *unmap_op;
> +                     struct page *scratch_page = get_balloon_scratch_page();
> +                     unsigned long scratch_page_address = (unsigned long)
> +                             __va(page_to_pfn(scratch_page) << PAGE_SHIFT);
>  
>                       /*
>                        * It might be that we queued all the m2p grant table
> @@ -990,21 +994,25 @@ int m2p_remove_override(struct page *page,
>                       }
>  
>                       mcs = xen_mc_entry(
> -                                     sizeof(struct gnttab_unmap_grant_ref));
> +                                     sizeof(struct 
> gnttab_unmap_and_replace));
>                       unmap_op = mcs.args;
>                       unmap_op->host_addr = kmap_op->host_addr;
> +                     unmap_op->new_addr = scratch_page_address;
>                       unmap_op->handle = kmap_op->handle;
> -                     unmap_op->dev_bus_addr = 0;
>  
>                       MULTI_grant_table_op(mcs.mc,
> -                                     GNTTABOP_unmap_grant_ref, unmap_op, 1);
> +                                     GNTTABOP_unmap_and_replace, unmap_op, 
> 1);
>  
>                       xen_mc_issue(PARAVIRT_LAZY_MMU);
>  
> -                     set_pte_at(&init_mm, address, ptep,
> -                                     pfn_pte(pfn, PAGE_KERNEL));
> -                     __flush_tlb_single(address);
> +                     mcs = __xen_mc_entry(0);
> +                     MULTI_update_va_mapping(mcs.mc, scratch_page_address,
> +                                     
> pfn_pte(page_to_pfn(get_balloon_scratch_page()),
> +                                     PAGE_KERNEL_RO), 0);
> +                     xen_mc_issue(PARAVIRT_LAZY_MMU);
> +
>                       kmap_op->host_addr = 0;
> +                     put_balloon_scratch_page();
>               }
>       }
>  
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 3c8803f..51f4c95 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -270,19 +270,12 @@ static int map_grant_pages(struct grant_map *map)
>                * with find_grant_ptes.
>                */
>               for (i = 0; i < map->count; i++) {
> -                     unsigned level;
>                       unsigned long address = (unsigned long)
>                               pfn_to_kaddr(page_to_pfn(map->pages[i]));
> -                     pte_t *ptep;
> -                     u64 pte_maddr = 0;
>                       BUG_ON(PageHighMem(map->pages[i]));
>  
> -                     ptep = lookup_address(address, &level);
> -                     pte_maddr = arbitrary_virt_to_machine(ptep).maddr;
> -                     gnttab_set_map_op(&map->kmap_ops[i], pte_maddr,
> -                             map->flags |
> -                             GNTMAP_host_map |
> -                             GNTMAP_contains_pte,
> +                     gnttab_set_map_op(&map->kmap_ops[i], address,
> +                             map->flags | GNTMAP_host_map,
>                               map->grants[i].ref,
>                               map->grants[i].domid);
>               }
> -- 
> 1.7.2.5
> 

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