[PATCH v2 2/6] x86/gnttab: do not implement GNTTABOP_cache_flush

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

The current underlying implementation of GNTTABOP_cache_flush on x86 won't
work as expected.  The provided {clean,invalidate}_dcache_va_range()
helpers only do a local pCPU cache flush, so the cache of previous pCPUs
where the vCPU might have run are not flushed.

However instead of attempting to fix this, make the GNTTABOP_cache_flush
operation only available to ARM.  There are no known users on x86, the
implementation is broken, and other architectures don't have grant-table
support yet.

With that operation not implemented on x86, the related
{clean,invalidate}_dcache_va_range() helpers can also be removed.

Fixes: f62dc81c2df7 ("x86: introduce more cache maintenance operations")
Fixes: 18e8d22fe750 ("introduce GNTTABOP_cache_flush")
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - Introduce Kconfig option.
 - Introduce CHANGELOG entry.
---
 CHANGELOG.md                        |  3 +++
 xen/arch/x86/include/asm/flushtlb.h | 19 -------------------
 xen/common/Kconfig                  |  5 +++++
 xen/common/grant_table.c            |  6 ++++++
 4 files changed, 14 insertions(+), 19 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1ea06524db20..21d7be0aa389 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -24,6 +24,9 @@ The format is based on [Keep a 
Changelog](https://keepachangelog.com/en/1.0.0/)
     - Ability to enable stack protector
 
 ### Removed
+ - On x86:
+   - GNTTABOP_cache_flush: it's unused on x86 and the implementation is
+     broken.
 
 ## 
[4.20.0](https://xenbits.xenproject.org/gitweb/?p=xen.git;a=shortlog;h=RELEASE-4.20.0)
 - 2025-03-05
 
diff --git a/xen/arch/x86/include/asm/flushtlb.h 
b/xen/arch/x86/include/asm/flushtlb.h
index 209ea1e62fae..cd625f911436 100644
--- a/xen/arch/x86/include/asm/flushtlb.h
+++ b/xen/arch/x86/include/asm/flushtlb.h
@@ -184,25 +184,6 @@ void flush_area_mask(const cpumask_t *mask, const void *va,
 }
 
 static inline void flush_page_to_ram(unsigned long mfn, bool sync_icache) {}
-static inline int invalidate_dcache_va_range(const void *p,
-                                             unsigned long size)
-{ return -EOPNOTSUPP; }
-static inline int clean_and_invalidate_dcache_va_range(const void *p,
-                                                       unsigned long size)
-{
-    unsigned int order = get_order_from_bytes(size);
-    /* sub-page granularity support needs to be added if necessary */
-    flush_area_local(p, FLUSH_CACHE|FLUSH_ORDER(order));
-    return 0;
-}
-static inline int clean_dcache_va_range(const void *p, unsigned long size)
-{
-    unsigned int order = get_order_from_bytes(size);
-
-    /* sub-page granularity support needs to be added if necessary */
-    flush_area_local(p, FLUSH_CACHE_WRITEBACK | FLUSH_ORDER(order));
-    return 0;
-}
 
 unsigned int guest_flush_tlb_flags(const struct domain *d);
 void guest_flush_tlb_mask(const struct domain *d, const cpumask_t *mask);
diff --git a/xen/common/Kconfig b/xen/common/Kconfig
index 6d43be2e6e8a..563b036474c0 100644
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -35,6 +35,11 @@ config GRANT_TABLE
 
          If unsure, say Y.
 
+config HAS_GRANT_CACHE_FLUSH
+       bool
+       depends on GRANT_TABLE
+       default ARM
+
 config EVTCHN_FIFO
        bool "Event Channel Fifo support" if EXPERT
        default y
diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index e75ff98aff1c..cf131c43a1f1 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -940,6 +940,7 @@ static void reduce_status_for_pin(struct domain *rd,
         gnttab_clear_flags(rd, clear_flags, status);
 }
 
+#ifdef CONFIG_HAS_GRANT_CACHE_FLUSH
 static struct active_grant_entry *grant_map_exists(const struct domain *ld,
                                                    struct grant_table *rgt,
                                                    mfn_t mfn,
@@ -975,6 +976,7 @@ static struct active_grant_entry *grant_map_exists(const 
struct domain *ld,
 
     return ERR_PTR(-EINVAL);
 }
+#endif /* CONFIG_HAS_GRANT_CACHE_FLUSH */
 
 union maptrack_node {
     struct {
@@ -3520,6 +3522,7 @@ 
gnttab_swap_grant_ref(XEN_GUEST_HANDLE_PARAM(gnttab_swap_grant_ref_t) uop,
     return 0;
 }
 
+#ifdef CONFIG_HAS_GRANT_CACHE_FLUSH
 static int _cache_flush(const gnttab_cache_flush_t *cflush, grant_ref_t 
*cur_ref)
 {
     struct domain *d, *owner;
@@ -3631,6 +3634,7 @@ 
gnttab_cache_flush(XEN_GUEST_HANDLE_PARAM(gnttab_cache_flush_t) uop,
 
     return 0;
 }
+#endif /* CONFIG_HAS_GRANT_CACHE_FLUSH */
 
 long do_grant_table_op(
     unsigned int cmd, XEN_GUEST_HANDLE_PARAM(void) uop, unsigned int count)
@@ -3773,6 +3777,7 @@ long do_grant_table_op(
         break;
     }
 
+#ifdef CONFIG_HAS_GRANT_CACHE_FLUSH
     case GNTTABOP_cache_flush:
     {
         XEN_GUEST_HANDLE_PARAM(gnttab_cache_flush_t) cflush =
@@ -3789,6 +3794,7 @@ long do_grant_table_op(
         }
         break;
     }
+#endif /* CONFIG_HAS_GRANT_CACHE_FLUSH */
 
     default:
         rc = -ENOSYS;
-- 
2.48.1