Re: [Xen-devel] [PATCH V3 8/8] xen: Speed up set_phys_to_machine() by using read-only mappings

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

On Tue, Nov 11, 2014 at 06:43:46AM +0100, Juergen Gross wrote:
> Instead of checking at each call of set_phys_to_machine() whether a
> new p2m page has to be allocated due to writing an entry in a large
> invalid or identity area, just map those areas read only and react
> to a page fault on write by allocating the new page.
> 
> This change will make the common path with no allocation much
> faster as it only requires a single write of the new mfn instead
> of walking the address translation tables and checking for the
> special cases.
> 
> Suggested-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Clever!

Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> ---
>  arch/x86/xen/p2m.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index 7df446d..58cf04c 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -70,6 +70,7 @@
>  
>  #include <asm/cache.h>
>  #include <asm/setup.h>
> +#include <asm/uaccess.h>
>  
>  #include <asm/xen/page.h>
>  #include <asm/xen/hypercall.h>
> @@ -313,9 +314,9 @@ static void __init xen_rebuild_p2m_list(unsigned long 
> *p2m)
>       paravirt_alloc_pte(&init_mm, __pa(p2m_identity_pte) >> PAGE_SHIFT);
>       for (i = 0; i < PTRS_PER_PTE; i++) {
>               set_pte(p2m_missing_pte + i,
> -                     pfn_pte(PFN_DOWN(__pa(p2m_missing)), PAGE_KERNEL));
> +                     pfn_pte(PFN_DOWN(__pa(p2m_missing)), PAGE_KERNEL_RO));
>               set_pte(p2m_identity_pte + i,
> -                     pfn_pte(PFN_DOWN(__pa(p2m_identity)), PAGE_KERNEL));
> +                     pfn_pte(PFN_DOWN(__pa(p2m_identity)), PAGE_KERNEL_RO));
>       }
>  
>       for (pfn = 0; pfn < xen_max_p2m_pfn; pfn += chunk) {
> @@ -362,7 +363,7 @@ static void __init xen_rebuild_p2m_list(unsigned long 
> *p2m)
>                               p2m_missing : p2m_identity;
>                       ptep = populate_extra_pte((unsigned long)(p2m + pfn));
>                       set_pte(ptep,
> -                             pfn_pte(PFN_DOWN(__pa(mfns)), PAGE_KERNEL));
> +                             pfn_pte(PFN_DOWN(__pa(mfns)), PAGE_KERNEL_RO));
>                       continue;
>               }
>  
> @@ -621,6 +622,9 @@ bool __set_phys_to_machine(unsigned long pfn, unsigned 
> long mfn)
>               return true;
>       }
>  
> +     if (likely(!__put_user(mfn, xen_p2m_addr + pfn)))
> +             return true;
> +
>       ptep = lookup_address((unsigned long)(xen_p2m_addr + pfn), &level);
>       BUG_ON(!ptep || level != PG_LEVEL_4K);
>  
> @@ -630,9 +634,7 @@ bool __set_phys_to_machine(unsigned long pfn, unsigned 
> long mfn)
>       if (pte_pfn(*ptep) == PFN_DOWN(__pa(p2m_identity)))
>               return mfn == IDENTITY_FRAME(pfn);
>  
> -     xen_p2m_addr[pfn] = mfn;
> -
> -     return true;
> +     return false;
>  }
>  
>  bool set_phys_to_machine(unsigned long pfn, unsigned long mfn)
> -- 
> 2.1.2
> 

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