Re: [PATCH v4 5/9] x86/mem-sharing: copy GADDR based shared guest areas

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Sep 28, 2023 at 09:16:20AM +0200, Jan Beulich wrote:
> In preparation of the introduction of new vCPU operations allowing to
> register the respective areas (one of the two is x86-specific) by
> guest-physical address, add the necessary fork handling (with the
> backing function yet to be filled in).
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> v3: Extend comment.
> 
> --- a/xen/arch/x86/mm/mem_sharing.c
> +++ b/xen/arch/x86/mm/mem_sharing.c
> @@ -1641,6 +1641,68 @@ static void copy_vcpu_nonreg_state(struc
>      hvm_set_nonreg_state(cd_vcpu, &nrs);
>  }
>  
> +static int copy_guest_area(struct guest_area *cd_area,
> +                           const struct guest_area *d_area,
> +                           struct vcpu *cd_vcpu,
> +                           const struct domain *d)
> +{
> +    mfn_t d_mfn, cd_mfn;
> +
> +    if ( !d_area->pg )
> +        return 0;
> +
> +    d_mfn = page_to_mfn(d_area->pg);
> +
> +    /* Allocate & map a page for the area if it hasn't been already. */
> +    if ( !cd_area->pg )
> +    {
> +        gfn_t gfn = mfn_to_gfn(d, d_mfn);
> +        struct p2m_domain *p2m = p2m_get_hostp2m(cd_vcpu->domain);
> +        p2m_type_t p2mt;
> +        p2m_access_t p2ma;
> +        unsigned int offset;
> +        int ret;
> +
> +        cd_mfn = p2m->get_entry(p2m, gfn, &p2mt, &p2ma, 0, NULL, NULL);
> +        if ( mfn_eq(cd_mfn, INVALID_MFN) )
> +        {
> +            struct page_info *pg = alloc_domheap_page(cd_vcpu->domain, 0);
> +
> +            if ( !pg )
> +                return -ENOMEM;
> +
> +            cd_mfn = page_to_mfn(pg);
> +            set_gpfn_from_mfn(mfn_x(cd_mfn), gfn_x(gfn));
> +
> +            ret = p2m->set_entry(p2m, gfn, cd_mfn, PAGE_ORDER_4K, p2m_ram_rw,
> +                                 p2m->default_access, -1);
> +            if ( ret )
> +                return ret;
> +        }

I'm still unsure why map_guest_area() shouldn't be able to deal with a
forked child needing the page to be mapped.  What happens when a
forked child executes the hypercall to map such areas against not yet
populated gfns?

Shouldn't map_guest_area() be capable of handling those calls and
populating on demand?

> +        else if ( p2mt != p2m_ram_rw )
> +            return -EBUSY;
> +
> +        /*
> +         * Map the area into the guest. For simplicity specify the entire range
> +         * up to the end of the page: All the function uses it for is to check
> +         * that the range doesn't cross page boundaries. Having the area mapped
> +         * in the original domain implies that it fits there and therefore will
> +         * also fit in the clone.
> +         */
> +        offset = PAGE_OFFSET(d_area->map);
> +        ret = map_guest_area(cd_vcpu, gfn_to_gaddr(gfn) + offset,
> +                             PAGE_SIZE - offset, cd_area, NULL);
> +        if ( ret )
> +            return ret;
> +    }
> +    else
> +        cd_mfn = page_to_mfn(cd_area->pg);
> +
> +    copy_domain_page(cd_mfn, d_mfn);

I think the page copy should be done only once, when the page is
populated on the child p2m.  Otherwise areas smaller than a page size
(like vpcu_time_info_t) that share the same page will get multiple
copies of the same data for no reason.

Thanks, Roger.