Re: [Xen-devel] [PATCH 02/13] xen: anchor linear p2m list in shared info structure

From: Juergen Gross
Date: Wed Feb 18 2015 - 05:57:51 EST

On 02/18/2015 11:54 AM, David Vrabel wrote:
On 18/02/15 10:50, Andrew Cooper wrote:
On 18/02/15 10:42, Juergen Gross wrote:

/* Set up p2m_top to point to the domain-builder provided p2m
pages */
@@ -469,8 +473,10 @@ static pte_t *alloc_p2m_pmd(unsigned long addr,
pte_t *pte_pg)

ptechk = lookup_address(vaddr, &level);
if (ptechk == pte_pg) {
+ HYPERVISOR_shared_info->arch.p2m_generation++;
__pmd(__pa(pte_newpg[i]) | _KERNPG_TABLE));
+ HYPERVISOR_shared_info->arch.p2m_generation++;

Do these increments of p2m_generation need to be atomic?

Hmm, they are done under lock. I don't think the compiler is allowed to
reorder the writes to p2m_generation across set_pmd().

They do need smp_wmb() to guarantee that the increment is visible before
the update occurs, just as the toolstack will need smp_rmb() to read.

smp_wmb() isn't good enough since you need the barrier even on non-smp
-- you need a wmb().

Okay, will do.


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