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 <jbeul...@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.
