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Re: [Xen-devel] [PATCH v10 5/6] x86/ioreq server: Asynchronously reset outstanding p2m_ioreq_server entries.





On 4/6/2017 3:48 PM, Jan Beulich wrote:
On 05.04.17 at 20:04, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -288,6 +288,10 @@ int guest_remove_page(struct domain *d, unsigned long gmfn)
           put_gfn(d, gmfn);
           return 1;
       }
+    if ( unlikely(p2mt == p2m_ioreq_server) )
+        p2m_change_type_one(d, gmfn,
+                            p2m_ioreq_server, p2m_ram_rw);
+
   #else
       mfn = gfn_to_mfn(d, _gfn(gmfn));
   #endif
To be honest, this looks more like a quick hack than a proper solution
at the first glance. To me it would seem preferable if the count was

Yeah, right. :)

adjusted at the point the P2M entry is being replaced (i.e. down the
call stack from guest_physmap_remove_page()). The closer to the
actual changing of the P2M entry, the less likely you miss any call
paths (as was already explained by George while suggesting to put
the accounting into the leaf most EPT/NPT functions).

Well, I thought I have explained the reason why I have always been hesitating to do the count in
atomic_write_ept_entry(). But seems I did not make it clear:

1> atomic_write_ept_entry() is used each time an p2m entry is written, but sweeping p2m_ioreq_server is only supposed to happen when an ioreq server unmaps. Checking the p2m here impacts the performance, and I do not think it is worthy - consider there may be very limited usage requirement for the p2m_ioreq_server;

2> atomic_write_ept_entry() does not have a p2m parameter, even some of its caller does not have either;

3> the lazy p2m type change is triggered at p2m level, by p2m_change_entry_type_global(). It can be used not only for intel ept. And supporting the count at the lowest level for non intel ept platform is more complex than
changes in atomic_write_ept_entry().

4> Fortunately, we have both resolve_misconfig() and do_recalc(), so I thought it would be enough to do the count
in these two routines, together with the count in p2m_change_type_one().

But I had to admit I did not think of the extreme scenarios raised by George - I had always assumed an p2m_ioreq_server
page will not be allocated to the balloon driver when it is in use.

So here I have another proposal - we shall not allow a p2m_ioreq_server being ballooned out. I mean, if some bug in kernel really allocates a p2m_ioreq_server page to a balloon driver, or if the driver is a malicious one which does not tell the device model this gfn shall be no longer emulated, the hypervisor shall let the ballooning fail for this gfn. After all, if such situation happens, the guest or the device model already have bugs, and these last 2 patches are to make sure that even if there's bug in guest/device model, xen will help do the cleanup, instead of to tolerate guest bugs.

If you think this is reasonable, I have drafted a patch, like this:

diff --git a/xen/arch/x86/mm/p2m-pod.c b/xen/arch/x86/mm/p2m-pod.c
index d5fea72..ff726ad 100644
--- a/xen/arch/x86/mm/p2m-pod.c
+++ b/xen/arch/x86/mm/p2m-pod.c
@@ -606,7 +606,8 @@ p2m_pod_decrease_reservation(struct domain *d,
             BUG_ON(p2m->pod.entry_count < 0);
             pod -= n;
         }
-        else if ( steal_for_cache && p2m_is_ram(t) )
+        else if ( steal_for_cache && p2m_is_ram(t) &&
+                  (t != p2m_ioreq_server) )
         {
             /*
              * If we need less than 1 << cur_order, we may end up stealing
diff --git a/xen/common/memory.c b/xen/common/memory.c
index 7dbddda..40d5545 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -288,6 +288,14 @@ int guest_remove_page(struct domain *d, unsigned long gmfn)
         put_gfn(d, gmfn);
         return 1;
     }
+    if ( unlikely(p2mt == p2m_ioreq_server) )
+    {
+        put_gfn(d, gmfn);
+        gdprintk(XENLOG_INFO, "Domain %u page %lx cannot be removed.\n",
+                d->domain_id, gmfn);
+        return 0;
+    }
+
 #else
     mfn = gfn_to_mfn(d, _gfn(gmfn));
 #endif

Changes made in p2m_pod_decrease_reservation() is to not let balloon driver to steal a p2m_ioreq_server page in PoD; Changes made in guest_remove_page() is to disallow a p2m_ioreq_server page being removed.


Thanks
Yu

Jan




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