[Xen-devel] [PATCH v4 6/6] domain: use PGC_extra domheap page for shared_info

[<paul@xxxxxxx>]

From: Paul Durrant <pdurrant@xxxxxxxxxx>

Currently shared_info is a shared xenheap page but shared xenheap pages
complicate future plans for live-update of Xen so it is desirable to,
where possible, not use them [1]. This patch therefore converts shared_info
into a PGC_extra domheap page. This does entail freeing shared_info during
domain_relinquish_resources() rather than domain_destroy() so care is
needed to avoid de-referencing a NULL shared_info pointer hence some
extra checks of 'is_dying' are needed.

NOTE: For Arm, the call to free_shared_info() in arch_domain_destroy() is
      left in place since it is idempotent and called in the error path for
      arch_domain_create().

[1] See 
https://lists.xenproject.org/archives/html/xen-devel/2020-02/msg02018.html

Signed-off-by: Paul Durrant <paul@xxxxxxx>
---
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Julien Grall <julien@xxxxxxx>
Cc: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Wei Liu <wl@xxxxxxx>

v2:
 - Addressed comments from Julien
 - Expanded the commit comment to explain why this patch is wanted
---
 xen/arch/arm/domain.c      |  2 ++
 xen/arch/x86/domain.c      |  3 ++-
 xen/common/domain.c        | 28 ++++++++++++++++++++++++----
 xen/common/event_channel.c |  3 +++
 xen/common/time.c          | 15 +++++++++++++++
 5 files changed, 46 insertions(+), 5 deletions(-)

diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
index 5298d80bd2..741f6dd444 100644
--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -1005,6 +1005,8 @@ int domain_relinquish_resources(struct domain *d)
         BUG();
     }
 
+    free_shared_info(d);
+
     return 0;
 }
 
diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index f6ed25e8ee..d962835f33 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -693,7 +693,6 @@ void arch_domain_destroy(struct domain *d)
         pv_domain_destroy(d);
     free_perdomain_mappings(d);
 
-    free_shared_info(d);
     cleanup_domain_irq_mapping(d);
 
     psr_domain_free(d);
@@ -2249,6 +2248,8 @@ int domain_relinquish_resources(struct domain *d)
     if ( is_hvm_domain(d) )
         hvm_domain_relinquish_resources(d);
 
+    free_shared_info(d);
+
     return 0;
 }
 
diff --git a/xen/common/domain.c b/xen/common/domain.c
index ba7a905258..886206f648 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -1650,24 +1650,44 @@ int continue_hypercall_on_cpu(
 
 int alloc_shared_info(struct domain *d, unsigned int memflags)
 {
-    if ( (d->shared_info.virt = alloc_xenheap_pages(0, memflags)) == NULL )
+    struct page_info *pg;
+
+    pg = alloc_domheap_page(d, MEMF_no_refcount | memflags);
+    if ( !pg )
         return -ENOMEM;
 
-    d->shared_info.mfn = virt_to_mfn(d->shared_info.virt);
+    if ( !get_page_and_type(pg, d, PGT_writable_page) )
+    {
+        /*
+         * The domain should not be running at this point so there is
+         * no way we should reach this error path.
+         */
+        ASSERT_UNREACHABLE();
+        return -ENODATA;
+    }
+
+    d->shared_info.mfn = page_to_mfn(pg);
+    d->shared_info.virt = __map_domain_page_global(pg);
 
     clear_page(d->shared_info.virt);
-    share_xen_page_with_guest(mfn_to_page(d->shared_info.mfn), d, SHARE_rw);
 
     return 0;
 }
 
 void free_shared_info(struct domain *d)
 {
+    struct page_info *pg;
+
     if ( !d->shared_info.virt )
         return;
 
-    free_xenheap_page(d->shared_info.virt);
+    unmap_domain_page_global(d->shared_info.virt);
     d->shared_info.virt = NULL;
+
+    pg = mfn_to_page(d->shared_info.mfn);
+
+    put_page_alloc_ref(pg);
+    put_page_and_type(pg);
 }
 
 /*
diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
index e86e2bfab0..a17422284d 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -1325,6 +1325,9 @@ void evtchn_destroy(struct domain *d)
 {
     unsigned int i;
 
+    /* This must be done before shared_info is freed */
+    BUG_ON(!d->shared_info.virt);
+
     /* After this barrier no new event-channel allocations can occur. */
     BUG_ON(!d->is_dying);
     spin_barrier(&d->event_lock);
diff --git a/xen/common/time.c b/xen/common/time.c
index 58fa9abc40..ada02faf07 100644
--- a/xen/common/time.c
+++ b/xen/common/time.c
@@ -99,6 +99,18 @@ void update_domain_wallclock_time(struct domain *d)
     uint32_t *wc_version;
     uint64_t sec;
 
+    if ( d != current->domain )
+    {
+        /*
+         * We need to check is_dying here as, if it is set, the
+         * shared_info may have been freed. To do this safely we need
+         * hold the domain lock.
+         */
+        domain_lock(d);
+        if ( d->is_dying )
+            goto unlock;
+    }
+
     spin_lock(&wc_lock);
 
     wc_version = &shared_info(d, wc_version);
@@ -121,6 +133,9 @@ void update_domain_wallclock_time(struct domain *d)
     *wc_version = version_update_end(*wc_version);
 
     spin_unlock(&wc_lock);
+ unlock:
+    if ( d != current->domain )
+        domain_unlock(d);
 }
 
 /* Set clock to <secs,usecs> after 00:00:00 UTC, 1 January, 1970. */
-- 
2.20.1


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