Re: [Xen-devel] [PATCH] x86/altp2m: cleanup p2m_altp2m_lazy_copy

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 12/04/2019 17:39, Tamas K Lengyel wrote:
> The p2m_altp2m_lazy_copy is responsible for lazily populating an altp2m view
> when the guest traps out due to no EPT entry being present in the active view.
> Currently the function took several inputs that it didn't use and also
> locked/unlocked gfns when it didn't need to.
>
> Signed-off-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>

Re-reveiwing in light of the discussion.

> diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
> index b9bbb8f485..140c707348 100644
> --- a/xen/arch/x86/mm/p2m.c
> +++ b/xen/arch/x86/mm/p2m.c
> @@ -2375,54 +2375,46 @@ bool_t p2m_switch_vcpu_altp2m_by_id(struct vcpu *v, 
> unsigned int idx)
>   *     indicate that outer handler should handle fault
>   */
>  
> -bool_t p2m_altp2m_lazy_copy(struct vcpu *v, paddr_t gpa,
> -                            unsigned long gla, struct npfec npfec,
> -                            struct p2m_domain **ap2m)
> +bool_t p2m_altp2m_lazy_copy(struct p2m_domain *ap2m, unsigned long gfn_l,
> +                            mfn_t hmfn, p2m_type_t hp2mt, p2m_access_t hp2ma,
> +                            unsigned int page_order)
>  {
> -    struct p2m_domain *hp2m = p2m_get_hostp2m(v->domain);
> -    p2m_type_t p2mt;
> -    p2m_access_t p2ma;
> -    unsigned int page_order;
> -    gfn_t gfn = _gfn(paddr_to_pfn(gpa));
> +    p2m_type_t ap2mt;
> +    p2m_access_t ap2ma;
>      unsigned long mask;
> -    mfn_t mfn;
> +    gfn_t gfn;
> +    mfn_t amfn;
>      int rv;
>  
> -    *ap2m = p2m_get_altp2m(v);
> -
> -    mfn = get_gfn_type_access(*ap2m, gfn_x(gfn), &p2mt, &p2ma,
> -                              0, &page_order);
> -    __put_gfn(*ap2m, gfn_x(gfn));
> -
> -    if ( !mfn_eq(mfn, INVALID_MFN) )
> -        return 0;

With the rearranged logic, and feeding in some of my cleanup series, you
want something like this:

/* Entry not present in hp2m?  Caller should handle the fault. */
if ( mfn_eq(hmfn, INVALID_MFN) )
    return 0;

There is no point doing the ap2m lookup if you're going to bail because
of the hp2m miss anyway.

> +    p2m_lock(ap2m);
>  
> -    mfn = get_gfn_type_access(hp2m, gfn_x(gfn), &p2mt, &p2ma,
> -                              P2M_ALLOC, &page_order);
> -    __put_gfn(hp2m, gfn_x(gfn));
> +    amfn = __get_gfn_type_access(ap2m, gfn_l, &ap2mt, &ap2ma,
> +                                 0, NULL, false);
>  
> -    if ( mfn_eq(mfn, INVALID_MFN) )
> +    /* Bail if entry is already in altp2m or there is no entry is hostp2m */

This comment then reduces to:

/* Entry already present in ap2m?  Caller should handle the fault. */

> +    if ( !mfn_eq(amfn, INVALID_MFN) || mfn_eq(hmfn, INVALID_MFN) )
> +    {
> +        p2m_unlock(ap2m);
>          return 0;
> -
> -    p2m_lock(*ap2m);
> +    }
>  
>      /*
>       * If this is a superpage mapping, round down both frame numbers
>       * to the start of the superpage.
>       */
>      mask = ~((1UL << page_order) - 1);
> -    mfn = _mfn(mfn_x(mfn) & mask);
> -    gfn = _gfn(gfn_x(gfn) & mask);
> +    hmfn = _mfn(mfn_x(hmfn) & mask);
> +    gfn = _gfn(gfn_l & mask);
>  
> -    rv = p2m_set_entry(*ap2m, gfn, mfn, page_order, p2mt, p2ma);
> -    p2m_unlock(*ap2m);
> +    rv = p2m_set_entry(ap2m, gfn, hmfn, page_order, hp2mt, hp2ma);
> +    p2m_unlock(ap2m);
>  
>      if ( rv )
>      {
>          gdprintk(XENLOG_ERR,
> -         "failed to set entry for %#"PRIx64" -> %#"PRIx64" p2m %#"PRIx64"\n",
> -         gfn_x(gfn), mfn_x(mfn), (unsigned long)*ap2m);
> -        domain_crash(hp2m->domain);
> +        "failed to set entry for %#"PRIx64" -> %#"PRIx64" p2m %#"PRIx64"\n",
> +        gfn_l, mfn_x(hmfn), (unsigned long)ap2m);

This should be a gprintk(), not a gdprintk(), because there is nothing
more infuriating in release builds than a domain_crash() with no
qualification.

Printing the virtual address of the ap2m is also useless as far as
diagnostics go.  How about something like:

"Failed to lazy copy gfn %"PRI_gfn" -> %"PRI_mfn" (type %u, access %u,
order %u) for view %u\n"

if there is an easy way to access the view id.  This at least stands a
chance of being useful when debugging.

> +        domain_crash(ap2m->domain);
>      }
>  
>      return 1;
> diff --git a/xen/include/asm-x86/p2m.h b/xen/include/asm-x86/p2m.h
> index 2801a8ccca..c25e2a3cd8 100644
> --- a/xen/include/asm-x86/p2m.h
> +++ b/xen/include/asm-x86/p2m.h
> @@ -867,8 +867,9 @@ void p2m_altp2m_check(struct vcpu *v, uint16_t idx);
>  void p2m_flush_altp2m(struct domain *d);
>  
>  /* Alternate p2m paging */
> -bool_t p2m_altp2m_lazy_copy(struct vcpu *v, paddr_t gpa,
> -    unsigned long gla, struct npfec npfec, struct p2m_domain **ap2m);
> +bool_t p2m_altp2m_lazy_copy(struct p2m_domain *ap2m, unsigned long gfn_l,
> +                            mfn_t hmfn, p2m_type_t hp2mt, p2m_access_t hp2ma,
> +                            unsigned int page_order);

Can we switch to bool while changing everything else as well?

Thanks,

~Andrew

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