Re: [Xen-devel] [PATCH 4/9] x86/np2m: Simplify nestedhvm_hap_nested_page_fault

[Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>]

One comment below.

On Fri, 2017-09-29 at 16:01 +0100, George Dunlap wrote:
> There is a possibility for nested_p2m to became stale between
> nestedhvm_hap_nested_page_fault() and nestedhap_fix_p2m().  At the moment
> this is handled by detecting such a race inside nestedhap_fix_p2m() and
> special-casing it.
> 
> Instead, introduce p2m_get_nestedp2m_locked(), which will returned a
> still-locked p2m.  This allows us to call nestedhap_fix_p2m() with the
> lock held and remove the code detecting the special-case.
> 
> Signed-off-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> ---
> v2:
> - Merged patch 9 and 10 ("x86/np2m: add p2m_get_nestedp2m_locked()"
>      and "x86/np2m: improve nestedhvm_hap_nested_page_fault()")
> - Updated commit message
> - Fix comment style in nestedhap_fix_p2m()
> 
> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> CC: Kevin Tian <kevin.tian@xxxxxxxxx>
> ---
>  xen/arch/x86/mm/hap/nested_hap.c | 31 +++++++++++++------------------
>  xen/arch/x86/mm/p2m.c            | 12 +++++++++---
>  xen/include/asm-x86/p2m.h        |  2 ++
>  3 files changed, 24 insertions(+), 21 deletions(-)
> 
> diff --git a/xen/arch/x86/mm/hap/nested_hap.c 
> b/xen/arch/x86/mm/hap/nested_hap.c
> index ed137fa784..844b32f702 100644
> --- a/xen/arch/x86/mm/hap/nested_hap.c
> +++ b/xen/arch/x86/mm/hap/nested_hap.c
> @@ -101,28 +101,23 @@ nestedhap_fix_p2m(struct vcpu *v, struct p2m_domain 
> *p2m,
>                    unsigned int page_order, p2m_type_t p2mt, p2m_access_t 
> p2ma)
>  {
>      int rc = 0;
> +    unsigned long gfn, mask;
> +    mfn_t mfn;
> +
>      ASSERT(p2m);
>      ASSERT(p2m->set_entry);
> +    ASSERT(p2m_locked_by_me(p2m));
>  
> -    p2m_lock(p2m);
> -
> -    /* If this p2m table has been flushed or recycled under our feet, 
> -     * leave it alone.  We'll pick up the right one as we try to 
> -     * vmenter the guest. */
> -    if ( p2m->np2m_base == nhvm_vcpu_p2m_base(v) )
> -    {
> -        unsigned long gfn, mask;
> -        mfn_t mfn;
> -
> -        /* If this is a superpage mapping, round down both addresses
> -         * to the start of the superpage. */
> -        mask = ~((1UL << page_order) - 1);
> +    /* 
> +     * If this is a superpage mapping, round down both addresses to
> +     * the start of the superpage.
> +     */
> +    mask = ~((1UL << page_order) - 1);
>  
> -        gfn = (L2_gpa >> PAGE_SHIFT) & mask;
> -        mfn = _mfn((L0_gpa >> PAGE_SHIFT) & mask);
> +    gfn = (L2_gpa >> PAGE_SHIFT) & mask;
> +    mfn = _mfn((L0_gpa >> PAGE_SHIFT) & mask);
>  
> -        rc = p2m_set_entry(p2m, gfn, mfn, page_order, p2mt, p2ma);
> -    }
> +    rc = p2m_set_entry(p2m, gfn, mfn, page_order, p2mt, p2ma);
>  
>      p2m_unlock(p2m);

I have the following fixup: move p2m_unlock() out of nestedhap_fix_p2m()
for balanced lock/unlock.

>  
> @@ -212,7 +207,6 @@ nestedhvm_hap_nested_page_fault(struct vcpu *v, paddr_t 
> *L2_gpa,
>      uint8_t p2ma_21 = p2m_access_rwx;
>  
>      p2m = p2m_get_hostp2m(d); /* L0 p2m */
> -    nested_p2m = p2m_get_nestedp2m(v);
>  
>      /* walk the L1 P2M table */
>      rv = nestedhap_walk_L1_p2m(v, *L2_gpa, &L1_gpa, &page_order_21, &p2ma_21,
> @@ -278,6 +272,7 @@ nestedhvm_hap_nested_page_fault(struct vcpu *v, paddr_t 
> *L2_gpa,
>      p2ma_10 &= (p2m_access_t)p2ma_21;
>  
>      /* fix p2m_get_pagetable(nested_p2m) */
> +    nested_p2m = p2m_get_nestedp2m_locked(v);
>      nestedhap_fix_p2m(v, nested_p2m, *L2_gpa, L0_gpa, page_order_20,
>          p2mt_10, p2ma_10);
>  
> diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
> index d3e602de22..aa3182dec6 100644
> --- a/xen/arch/x86/mm/p2m.c
> +++ b/xen/arch/x86/mm/p2m.c
> @@ -1813,7 +1813,7 @@ static void assign_np2m(struct vcpu *v, struct 
> p2m_domain *p2m)
>  }
>  
>  struct p2m_domain *
> -p2m_get_nestedp2m(struct vcpu *v)
> +p2m_get_nestedp2m_locked(struct vcpu *v)
>  {
>      struct nestedvcpu *nv = &vcpu_nestedhvm(v);
>      struct domain *d = v->domain;
> @@ -1838,7 +1838,6 @@ p2m_get_nestedp2m(struct vcpu *v)
>                  hvm_asid_flush_vcpu(v);
>              p2m->np2m_base = np2m_base;
>              assign_np2m(v, p2m);
> -            p2m_unlock(p2m);
>              nestedp2m_unlock(d);
>  
>              return p2m;
> @@ -1854,12 +1853,19 @@ p2m_get_nestedp2m(struct vcpu *v)
>      p2m->np2m_base = np2m_base;
>      hvm_asid_flush_vcpu(v);
>      assign_np2m(v, p2m);
> -    p2m_unlock(p2m);
>      nestedp2m_unlock(d);
>  
>      return p2m;
>  }
>  
> +struct p2m_domain *p2m_get_nestedp2m(struct vcpu *v)
> +{
> +    struct p2m_domain *p2m = p2m_get_nestedp2m_locked(v);
> +    p2m_unlock(p2m);
> +
> +    return p2m;
> +}
> +
>  struct p2m_domain *
>  p2m_get_p2m(struct vcpu *v)
>  {
> diff --git a/xen/include/asm-x86/p2m.h b/xen/include/asm-x86/p2m.h
> index 85874ab401..4a1c10c130 100644
> --- a/xen/include/asm-x86/p2m.h
> +++ b/xen/include/asm-x86/p2m.h
> @@ -363,6 +363,8 @@ struct p2m_domain {
>   * Updates vCPU's n2pm to match its np2m_base in VMCX12 and returns that 
> np2m.
>   */
>  struct p2m_domain *p2m_get_nestedp2m(struct vcpu *v);
> +/* Similar to the above except that returned p2m is still write-locked */
> +struct p2m_domain *p2m_get_nestedp2m_locked(struct vcpu *v);
>  
>  /* If vcpu is in host mode then behaviour matches p2m_get_hostp2m().
>   * If vcpu is in guest mode then behaviour matches p2m_get_nestedp2m().
-- 
Thanks,
Sergey
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