Re: [Xen-devel] [PATCH 13/13] Nested Virtualization: hap-on-hap

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

Hi, 

This patch doesn't address most of my concerns with the last version. 

My comments about callers of gva_to_gfn still stand -- basically,
gva_to_gfn should return an N1 GFN, and most callers (including hvm_copy)
should not need to know about N2 GFNs or multiple p2ms.

Also, you're still copying large parts of the existing HAP walker
(e.g. the next_level function).  Can you avoid the duplication by
using a write_p2m_entry() callback, since that seems to be the part
that's different?

Skipping a flush when p2m->cr3 == 0 is not safe.  I suggested using -1
as the magic value last time. 

My comments on why p2m_flush_locked() isn't enough to reclaim an in-use
p2m for recycling still stand.

I have one more comment on the new code:

At 18:45 +0000 on 12 Nov (1289587557), Christoph Egger wrote:
>  hap_write_p2m_entry(struct vcpu *v, unsigned long gfn, l1_pgentry_t *p,
>                      mfn_t table_mfn, l1_pgentry_t new, unsigned int level)
>  {
> +    struct domain *d = v->domain;
>      uint32_t old_flags;
>  
> -    hap_lock(v->domain);
> +    old_flags = l1e_get_flags(*p);
>  
> -    old_flags = l1e_get_flags(*p);
> +    /* We know always use the host p2m here, regardless if the vcpu
> +     * is in host or guest mode. The vcpu can be in guest mode by
> +     * a hypercall which passes a domain and chooses mostly the first
> +     * vcpu.
> +     * XXX This is the reason why this function can not be used re-used
> +     * for updating the nestedp2m. Otherwise, hypercalls would randomly
> +     * operate on host p2m and nested p2m.
> +     */
> +    if ( nestedhvm_enabled(d) ) {
> +        mfn_t omfn = _mfn(l1e_get_pfn(*p));
> +        p2m_type_t op2mt = p2m_flags_to_type(old_flags);
> +
> +        if ( p2m_is_valid(op2mt) && mfn_valid(omfn) ) {

I think you need to do this flush even if !mfn_valid(omfn) - for example
if you're removing a mapping of an MMIO area.

> +            mfn_t nmfn = _mfn(l1e_get_pfn(new));
> +            p2m_type_t np2mt = p2m_flags_to_type(l1e_get_flags(new));
> +
> +            if ( p2m_is_valid(np2mt)
> +                && mfn_valid(nmfn)
> +                && !(l1e_get_flags(new) & _PAGE_PRESENT) )

I don't understand this test at all - you need to flush if you're
removing an old present entry regardless of what replaces it. The only
case where you can skip the flush is if old == new.

Cheers,

Tim.


> +            {
> +                /* This GFN -> MFN is going to get removed. */
> +                /* XXX There is a more efficient way to do that
> +                 * but it works for now.
> +                 * Note, p2m_flush_nestedp2m calls hap_lock() internally.
> +                 */
> +                p2m_flush_nestedp2m(d);
> +            }
> +        }
> +    }
> +
> +    hap_lock(d);
> +

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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