[PATCH V5 22/22] xen/arm: Add mapcache invalidation handling

[Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

We need to send mapcache invalidation request to qemu/demu everytime
the page gets removed from a guest.

At the moment, the Arm code doesn't explicitely remove the existing
mapping before inserting the new mapping. Instead, this is done
implicitely by __p2m_set_entry().

So we need to recognize a case when old entry is a RAM page *and*
the new MFN is different in order to set the corresponding flag.
The most suitable place to do this is p2m_free_entry(), there
we can find the correct leaf type. The invalidation request
will be sent in do_trap_hypercall() later on.

Taking into the account the following the do_trap_hypercall()
is the best place to send invalidation request:
 - The only way a guest can modify its P2M on Arm is via an hypercall
 - When sending the invalidation request, the vCPU will be blocked
   until all the IOREQ servers have acknowledged the invalidation

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>
Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
[On Arm only]
Tested-by: Wei Chen <Wei.Chen@xxxxxxx>

---
Please note, this is a split/cleanup/hardening of Julien's PoC:
"Add support for Guest IO forwarding to a device emulator"

***
Please note, this patch depends on the following which is
on review:
https://patchwork.kernel.org/patch/11803383/

This patch is on par with x86 code (whether it is buggy or not).
If there is a need to improve/harden something, this can be done on
a follow-up.
***

Changes V1 -> V2:
   - new patch, some changes were derived from (+ new explanation):
     xen/ioreq: Make x86's invalidate qemu mapcache handling common
   - put setting of the flag into __p2m_set_entry()
   - clarify the conditions when the flag should be set
   - use domain_has_ioreq_server()
   - update do_trap_hypercall() by adding local variable

Changes V2 -> V3:
   - update patch description
   - move check to p2m_free_entry()
   - add a comment
   - use "curr" instead of "v" in do_trap_hypercall()

Changes V3 -> V4:
   - update patch description
   - re-order check in p2m_free_entry() to call domain_has_ioreq_server()
     only if p2m->domain == current->domain
   - add a comment in do_trap_hypercall()

Changes V4 -> V5:
   - add Stefano's R-b
   - update comment in do_trap_hypercall()
---
---
 xen/arch/arm/p2m.c   | 25 +++++++++++++++++--------
 xen/arch/arm/traps.c | 20 +++++++++++++++++---
 2 files changed, 34 insertions(+), 11 deletions(-)

diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
index d41c4fa..26acb95d 100644
--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -1,6 +1,7 @@
 #include <xen/cpu.h>
 #include <xen/domain_page.h>
 #include <xen/iocap.h>
+#include <xen/ioreq.h>
 #include <xen/lib.h>
 #include <xen/sched.h>
 #include <xen/softirq.h>
@@ -749,17 +750,25 @@ static void p2m_free_entry(struct p2m_domain *p2m,
     if ( !p2m_is_valid(entry) )
         return;
 
-    /* Nothing to do but updating the stats if the entry is a super-page. */
-    if ( p2m_is_superpage(entry, level) )
+    if ( p2m_is_superpage(entry, level) || (level == 3) )
     {
-        p2m->stats.mappings[level]--;
-        return;
-    }
+#ifdef CONFIG_IOREQ_SERVER
+        /*
+         * If this gets called (non-recursively) then either the entry
+         * was replaced by an entry with a different base (valid case) or
+         * the shattering of a superpage was failed (error case).
+         * So, at worst, the spurious mapcache invalidation might be sent.
+         */
+        if ( (p2m->domain == current->domain) &&
+              domain_has_ioreq_server(p2m->domain) &&
+              p2m_is_ram(entry.p2m.type) )
+            p2m->domain->mapcache_invalidate = true;
+#endif
 
-    if ( level == 3 )
-    {
         p2m->stats.mappings[level]--;
-        p2m_put_l3_page(entry);
+        /* Nothing to do if the entry is a super-page. */
+        if ( level == 3 )
+            p2m_put_l3_page(entry);
         return;
     }
 
diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index 4cdd343..64b740b 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -1443,6 +1443,7 @@ static void do_trap_hypercall(struct cpu_user_regs *regs, 
register_t *nr,
                               const union hsr hsr)
 {
     arm_hypercall_fn_t call = NULL;
+    struct vcpu *curr = current;
 
     BUILD_BUG_ON(NR_hypercalls < ARRAY_SIZE(arm_hypercall_table) );
 
@@ -1459,7 +1460,7 @@ static void do_trap_hypercall(struct cpu_user_regs *regs, 
register_t *nr,
         return;
     }
 
-    current->hcall_preempted = false;
+    curr->hcall_preempted = false;
 
     perfc_incra(hypercalls, *nr);
     call = arm_hypercall_table[*nr].fn;
@@ -1472,7 +1473,7 @@ static void do_trap_hypercall(struct cpu_user_regs *regs, 
register_t *nr,
     HYPERCALL_RESULT_REG(regs) = call(HYPERCALL_ARGS(regs));
 
 #ifndef NDEBUG
-    if ( !current->hcall_preempted )
+    if ( !curr->hcall_preempted )
     {
         /* Deliberately corrupt parameter regs used by this hypercall. */
         switch ( arm_hypercall_table[*nr].nr_args ) {
@@ -1489,8 +1490,21 @@ static void do_trap_hypercall(struct cpu_user_regs 
*regs, register_t *nr,
 #endif
 
     /* Ensure the hypercall trap instruction is re-executed. */
-    if ( current->hcall_preempted )
+    if ( curr->hcall_preempted )
         regs->pc -= 4;  /* re-execute 'hvc #XEN_HYPERCALL_TAG' */
+
+#ifdef CONFIG_IOREQ_SERVER
+    /*
+     * We call ioreq_signal_mapcache_invalidate from do_trap_hypercall()
+     * because the only way a guest can modify its P2M on Arm is via an
+     * hypercall.
+     * Note that sending the invalidation request causes the vCPU to block
+     * until all the IOREQ servers have acknowledged the invalidation.
+     */
+    if ( unlikely(curr->domain->mapcache_invalidate) &&
+         test_and_clear_bool(curr->domain->mapcache_invalidate) )
+        ioreq_signal_mapcache_invalidate();
+#endif
 }
 
 void arch_hypercall_tasklet_result(struct vcpu *v, long res)
-- 
2.7.4