Hi,Tim
 I have confusion when understanding monitor table in shadow
mode. In arch/x86/mm/shadow/mutli.c, the definition of
sh_make_monitor_table is following:
 #if SHADOW_PAGING_LEVELS == GUEST_PAGING_LEVELS
 mfn_t
 sh_make_monitor_table(struct vcpu *v)
 {
 .......
 }
The macro SHADOW_PAGING_LEVELS is defined in arch/x86/mm/shadow/types.h:
 #if GUEST_PAGING_LEVELS == 4
 #define SHADOW_PAGING_LEVELS 4
 #else
 #define SHADOW_PAGING_LEVELS 3
 #endif
If a 32-bit hvm is created, its GUEST_PAGING_LEVELS should be 2. In this
case, SHADOW_PAGING_LEVELS will be 3 and sh_make_monitor_table will not
be compiled. Is it correct? But, when a hvm domain is initialized, Xen
will call sh_update_paging_modes to create a monitor table for it:
In sh_update_paging_modes:
 #ifdef __x86_64__
 if ( hvm_long_mode_enabled(v) )
 {
 // long mode guest...
 v->arch.paging.mode =
 &SHADOW_INTERNAL_NAME(sh_paging_mode, 4);
 }
 else
 #endif
 if ( hvm_pae_enabled(v) )
 {
 // 32-bit PAE mode guest...
 v->arch.paging.mode =
 &SHADOW_INTERNAL_NAME(sh_paging_mode, 3);
 }
 else
 {
 // 32-bit 2 level guest...
 v->arch.paging.mode =
 &SHADOW_INTERNAL_NAME(sh_paging_mode, 2);
 }
 }
 if ( pagetable_is_null(v->arch.monitor_table) )
 {
 mfn_t mmfn =
v->arch.paging.mode->shadow.make_monitor_table(v);
 v->arch.monitor_table = pagetable_from_mfn(mmfn);
 make_cr3(v, mfn_x(mmfn));
 hvm_update_host_cr3(v);
 }
This function will finally be called by hvm_domain_initialise. But
sh_make_monitor_table is NULL-poinitor. What will happened in this case?
Need your help!
-Techie
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