[Xen-devel] [PATCH v8 1/5] x86/ioreq server: Release the p2m lock after mmio is handled.

[Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>]

Routine hvmemul_do_io() may need to peek the p2m type of a gfn to
select the ioreq server. For example, operations on gfns with
p2m_ioreq_server type will be delivered to a corresponding ioreq
server, and this requires that the p2m type not be switched back
to p2m_ram_rw during the emulation process. To avoid this race
condition, we delay the release of p2m lock in hvm_hap_nested_page_fault()
until mmio is handled.

Note: previously in hvm_hap_nested_page_fault(), put_gfn() was moved
before the handling of mmio, due to a deadlock risk between the p2m
lock and the event lock(in commit 77b8dfe). Later, a per-event channel
lock was introduced in commit de6acb7, to send events. So we do not
need to worry about the deadlock issue.

Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Cc: Paul Durrant <paul.durrant@xxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/hvm/hvm.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index ccfae4f..a9db7f7 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -1870,18 +1870,14 @@ int hvm_hap_nested_page_fault(paddr_t gpa, unsigned 
long gla,
          (npfec.write_access &&
           (p2m_is_discard_write(p2mt) || (p2mt == p2m_ioreq_server))) )
     {
-        __put_gfn(p2m, gfn);
-        if ( ap2m_active )
-            __put_gfn(hostp2m, gfn);
-
         rc = 0;
         if ( unlikely(is_pvh_domain(currd)) )
-            goto out;
+            goto out_put_gfn;
 
         if ( !handle_mmio_with_translation(gla, gpa >> PAGE_SHIFT, npfec) )
             hvm_inject_hw_exception(TRAP_gp_fault, 0);
         rc = 1;
-        goto out;
+        goto out_put_gfn;
     }
 
     /* Check if the page has been paged out */
-- 
1.9.1


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