[PATCH] x86/pv: limit GDT and LDT mappings areas to max number of vCPUs

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

The allocation of the paging structures in the per-domain area for mapping the
guest GDT and LDT can be limited to the maximum number of vCPUs the guest can
have.  The maximum number of vCPUs is available at domain creation since commit
4737fa52ce86.

Limiting to the actual number of vCPUs avoids wasting memory for paging
structures that will never be used.  Current logic unconditionally uses 513
pages, one page for the L3, plus 512 L1 pages.  For guests with equal or less
than 16 vCPUs only 2 pages are used (each guest vCPU GDT/LDT can only consume
32 L1 slots).

No functional change intended, all possible domain vCPUs should have the GDT
and LDT paging structures allocated and setup at domain creation, just like
before the change.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/pv/domain.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/pv/domain.c b/xen/arch/x86/pv/domain.c
index d5a8564c1cbe..e861e3ce71d9 100644
--- a/xen/arch/x86/pv/domain.c
+++ b/xen/arch/x86/pv/domain.c
@@ -346,7 +346,7 @@ void pv_domain_destroy(struct domain *d)
     pv_l1tf_domain_destroy(d);
 
     destroy_perdomain_mapping(d, GDT_LDT_VIRT_START,
-                              GDT_LDT_MBYTES << (20 - PAGE_SHIFT));
+                              d->max_vcpus << GDT_LDT_VCPU_SHIFT);
 
     XFREE(d->arch.pv.cpuidmasks);
 
@@ -377,7 +377,7 @@ int pv_domain_initialise(struct domain *d)
         goto fail;
 
     rc = create_perdomain_mapping(d, GDT_LDT_VIRT_START,
-                                  GDT_LDT_MBYTES << (20 - PAGE_SHIFT),
+                                  d->max_vcpus << GDT_LDT_VCPU_SHIFT,
                                   NULL, NULL);
     if ( rc )
         goto fail;
-- 
2.46.0