Re: [Xen-devel] [PATCH 05/10] VMX: add help functions to support PML

[Kai Huang <kai.huang@xxxxxxxxxxxxxxx>]

On 04/09/2015 08:00 PM, Tim Deegan wrote:
> Hi,
>
> At 10:35 +0800 on 27 Mar (1427452549), Kai Huang wrote:
> > +void vmx_vcpu_flush_pml_buffer(struct vcpu *v)
> > +{
> > +    uint64_t *pml_buf;
> > +    unsigned long pml_idx;
> > +
> > +    ASSERT(vmx_vcpu_pml_enabled(v));
> > +
> > +    vmx_vmcs_enter(v);
> > +
> > +    __vmread(GUEST_PML_INDEX, &pml_idx);
> > +
> > +    /* Do nothing if PML buffer is empty */
> > +    if ( pml_idx == (PML_ENTITY_NUM - 1) )
> > +        goto out;
> > +
> > +    pml_buf = map_domain_page(page_to_mfn(v->arch.hvm_vmx.pml_pg));
> > +
> > +    /*
> > +     * PML index can be either 2^16-1 (buffer is full), or 0~511 (buffer is not
> > +     * full), and in latter case PML index always points to next available
> > +     * entity.
> > +     */
> > +    if (pml_idx >= PML_ENTITY_NUM)
> > +        pml_idx = 0;
> > +    else
> > +        pml_idx++;
> > +
> > +    for ( ; pml_idx < PML_ENTITY_NUM; pml_idx++ )
> > +    {
> > +        struct p2m_domain *p2m = p2m_get_hostp2m(v->domain);
> > +        unsigned long gfn;
> > +        mfn_t mfn;
> > +        p2m_type_t t;
> > +        p2m_access_t a;
> > +
> > +        gfn = pml_buf[pml_idx] >> PAGE_SHIFT;
> > +        mfn = p2m->get_entry(p2m, gfn, &t, &a, 0, NULL);
> Please don't call p2m->get_entry() directly -- that interface should
> only be used inside the p2m code.  As it happens, I don't think this
> lookup is correct anyway: the logging only sees races (which are not
> interesting) or buggy hardware (which is not worth the extra lookup to
> detect).
>
> So you only need this to get 'mfn' to pass to paging_mark_dirty().
True.
> That's also buggy, because there's no locking here to make sure
> gfn->mfn->gfn ends up in the right place. :(
Yes you are right. Thanks for pointing out.

Just curious, looks if I use get_gfn_type_access and put_gfn here, the 
gfn->mfn->gfn can be guaranteed, and it's safe to use them here, right?
> I think the right thing to do is:
>
>   - split paging_park_dirty() into paging_mark_gfn_dirty() (the bulk of
>     the current function) and a paging_mark_dirty() wrapper that does
>     get_gpfn_from_mfn(mfn_x(gmfn)) and calls paging_mark_gfn_dirty().
>
>   - call paging_mark_gfn_dirty() from vmx_vcpu_flush_pml_buffer().
>
> That will avoid _two_ p2m lookups in this function. :)
Yours is indeed much better! Will do in this way.

Thanks,
-Kai
> Cheers,
>
> Tim.

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