diff mbox series

[14/22] x86/domain_page: remove the fast paths when mfn is not in the directmap

Message ID 20221216114853.8227-15-julien@xen.org (mailing list archive)
State New, archived
Headers show
Series Remove the directmap | expand

Commit Message

Julien Grall Dec. 16, 2022, 11:48 a.m. UTC 
From: Hongyan Xia <hongyxia@amazon.com>

When mfn is not in direct map, never use mfn_to_virt for any mappings.

We replace mfn_x(mfn) <= PFN_DOWN(__pa(HYPERVISOR_VIRT_END - 1)) with
arch_mfns_in_direct_map(mfn, 1) because these two are equivalent. The
extra comparison in arch_mfns_in_direct_map() looks different but because
DIRECTMAP_VIRT_END is always higher, it does not make any difference.

Lastly, domain_page_map_to_mfn() needs to gain to a special case for
the PMAP.

Signed-off-by: Hongyan Xia <hongyxia@amazon.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>

----
    Changes since Hongyan's version:
        * arch_mfn_in_direct_map() was renamed to arch_mfns_in_directmap()
        * add a special case for the PMAP in domain_page_map_to_mfn()
---
 xen/arch/x86/domain_page.c | 50 +++++++++++++++++++++++++++++++-------
 1 file changed, 41 insertions(+), 9 deletions(-)

Comments

Jan Beulich Jan. 11, 2023, 2:11 p.m. UTC | #1 
On 16.12.2022 12:48, Julien Grall wrote:
> From: Hongyan Xia <hongyxia@amazon.com>
> 
> When mfn is not in direct map, never use mfn_to_virt for any mappings.
> 
> We replace mfn_x(mfn) <= PFN_DOWN(__pa(HYPERVISOR_VIRT_END - 1)) with
> arch_mfns_in_direct_map(mfn, 1) because these two are equivalent. The
> extra comparison in arch_mfns_in_direct_map() looks different but because
> DIRECTMAP_VIRT_END is always higher, it does not make any difference.
> 
> Lastly, domain_page_map_to_mfn() needs to gain to a special case for
> the PMAP.
> 
> Signed-off-by: Hongyan Xia <hongyxia@amazon.com>
> Signed-off-by: Julien Grall <jgrall@amazon.com>

This looks plausible, but cannot really be fully judged upon before the
mapcache_current_vcpu() aspects pointed out earlier have been sorted.
As to using pmap - assuming you've done an audit and the number of
simultaneous mappings that can be in use can be proven to not exceed
the number of slots available, can you please say so in the description?
I have to admit though that I'm wary - this isn't a per-CPU number of
slots aiui, but a global one. But then you also have a BUG_ON() there
restricting the use to early boot. The reasoning for this is also
missing (and might address my concern).

Jan
Elias El Yandouzi Jan. 11, 2024, 2:22 p.m. UTC | #2 
Hi Jan,

On 11/01/2023 14:11, Jan Beulich wrote:
> As to using pmap - assuming you've done an audit and the number of
> simultaneous mappings that can be in use can be proven to not exceed
> the number of slots available, can you please say so in the description?

I don't know if any audit has been made but a similar code has been 
internally used for a few years now without any problem.
Quickly looking at the slots usage, I found that at most only 4 slots 
out of 8 would be used during boot time.

> I have to admit though that I'm wary - this isn't a per-CPU number of
> slots aiui, but a global one. But then you also have a BUG_ON() there
> restricting the use to early boot. The reasoning for this is also
> missing (and might address my concern).


Indeed, this isn't presented as a per-CPU number of slots, but the slots 
can be only used before we reach the state SYS_STATE_smp_boot. So 
effectively, only the BSP would use the PMAP slots.

The PMAP slots are meant to map at most the number of level page-tables.
See the comment on top of NUM_FIX_PMAP definition.
diff mbox series

Patch

diff --git a/xen/arch/x86/domain_page.c b/xen/arch/x86/domain_page.c
index 55e337aaf703..89caefc8a210 100644
--- a/xen/arch/x86/domain_page.c
+++ b/xen/arch/x86/domain_page.c
@@ -14,8 +14,10 @@ 
 #include <xen/sched.h>
 #include <xen/vmap.h>
 #include <asm/current.h>
+#include <asm/fixmap.h>
 #include <asm/flushtlb.h>
 #include <asm/hardirq.h>
+#include <asm/pmap.h>
 #include <asm/setup.h>
 
 static DEFINE_PER_CPU(struct vcpu *, override);
@@ -35,10 +37,11 @@  static inline struct vcpu *mapcache_current_vcpu(void)
     /*
      * When using efi runtime page tables, we have the equivalent of the idle
      * domain's page tables but current may point at another domain's VCPU.
-     * Return NULL as though current is not properly set up yet.
+     * Return the idle domains's vcpu on that core because the efi per-domain
+     * region (where the mapcache is) is in-sync with the idle domain.
      */
     if ( efi_rs_using_pgtables() )
-        return NULL;
+        return idle_vcpu[smp_processor_id()];
 
     /*
      * If guest_table is NULL, and we are running a paravirtualised guest,
@@ -77,18 +80,24 @@  void *map_domain_page(mfn_t mfn)
     struct vcpu_maphash_entry *hashent;
 
 #ifdef NDEBUG
-    if ( mfn_x(mfn) <= PFN_DOWN(__pa(HYPERVISOR_VIRT_END - 1)) )
+    if ( arch_mfns_in_directmap(mfn_x(mfn), 1) )
         return mfn_to_virt(mfn_x(mfn));
 #endif
 
     v = mapcache_current_vcpu();
-    if ( !v )
-        return mfn_to_virt(mfn_x(mfn));
+    if ( !v || !v->domain->arch.mapcache.inuse )
+    {
+        if ( arch_mfns_in_directmap(mfn_x(mfn), 1) )
+            return mfn_to_virt(mfn_x(mfn));
+        else
+        {
+            BUG_ON(system_state >= SYS_STATE_smp_boot);
+            return pmap_map(mfn);
+        }
+    }
 
     dcache = &v->domain->arch.mapcache;
     vcache = &v->arch.mapcache;
-    if ( !dcache->inuse )
-        return mfn_to_virt(mfn_x(mfn));
 
     perfc_incr(map_domain_page_count);
 
@@ -184,6 +193,12 @@  void unmap_domain_page(const void *ptr)
     if ( !va || va >= DIRECTMAP_VIRT_START )
         return;
 
+    if ( va >= FIXADDR_START && va < FIXADDR_TOP )
+    {
+        pmap_unmap((void *)ptr);
+        return;
+    }
+
     ASSERT(va >= MAPCACHE_VIRT_START && va < MAPCACHE_VIRT_END);
 
     v = mapcache_current_vcpu();
@@ -237,7 +252,7 @@  int mapcache_domain_init(struct domain *d)
     unsigned int bitmap_pages;
 
 #ifdef NDEBUG
-    if ( !mem_hotplug && max_page <= PFN_DOWN(__pa(HYPERVISOR_VIRT_END - 1)) )
+    if ( !mem_hotplug && arch_mfn_in_directmap(0, max_page) )
         return 0;
 #endif
 
@@ -308,7 +323,7 @@  void *map_domain_page_global(mfn_t mfn)
             local_irq_is_enabled()));
 
 #ifdef NDEBUG
-    if ( mfn_x(mfn) <= PFN_DOWN(__pa(HYPERVISOR_VIRT_END - 1)) )
+    if ( arch_mfn_in_directmap(mfn_x(mfn, 1)) )
         return mfn_to_virt(mfn_x(mfn));
 #endif
 
@@ -335,6 +350,23 @@  mfn_t domain_page_map_to_mfn(const void *ptr)
     if ( va >= DIRECTMAP_VIRT_START )
         return _mfn(virt_to_mfn(ptr));
 
+    /*
+     * The fixmap is stealing the top-end of the VMAP. So the check for
+     * the PMAP *must* happen first.
+     *
+     * Also, the fixmap translate a slot to an address backwards. The
+     * logic will rely on it to avoid any complexity. So check at
+     * compile time this will always hold.
+    */
+    BUILD_BUG_ON(fix_to_virt(FIX_PMAP_BEGIN) < fix_to_virt(FIX_PMAP_END));
+
+    if ( ((unsigned long)fix_to_virt(FIX_PMAP_END) <= va) &&
+         ((va & PAGE_MASK) <= (unsigned long)fix_to_virt(FIX_PMAP_BEGIN)) )
+    {
+        BUG_ON(system_state >= SYS_STATE_smp_boot);
+        return l1e_get_mfn(l1_fixmap[l1_table_offset(va)]);
+    }
+
     if ( va >= VMAP_VIRT_START && va < VMAP_VIRT_END )
         return vmap_to_mfn(va);