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

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Date:	Fri, 9 Aug 2013 11:26:01 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org,
	dcrisan@...xiant.com, alex@...x.org.uk, ian.campbell@...rix.com
Subject: Re: [PATCH v4 2/2] xen/m2p: use GNTTABOP_unmap_and_replace to
 reinstate the original mapping

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@...citrix.com>
> Reviewed-by: David Vrabel <david.vrabel@...rix.com>
> CC: alex@...x.org.uk
> CC: dcrisan@...xiant.com
> ---
>  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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