x86/pv: Move hv_compat_vstart into pv_domain

The hv_compat_vstart variable is hidden behind CONFIG_PV32 but lives in
arch_domain.  Moving it into pv_domain is an obvious improvement.

Its value however is less obvious, and is a mess.

In !PV32 builds, it's uniformly treated as 0, but in PV32 builds it's ~0U (HVM
guests), __HYPERVISOR_COMPAT_VIRT_START (PV guests), or custom (PV32 dom0).
This seems to work because users are guarded behind is_pv32_{domain,vcpu}().

Simplify things by leaving it as 0 in PV32 builds for most domains,
initialising it in only in switch_compat() when a domain becomes 32bit PV.
dom0_construct() adjusts the value after calling switch_compat().

Suggested-by: Grygorii Strashko <grygorii_strashko@epam.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/domain.c

@@ -890,11 +890,6 @@ int arch_domain_create(struct domain *d,
     }
     d->arch.emulation_flags = emflags;
 
-#ifdef CONFIG_PV32
-    d->arch.hv_compat_vstart =
-        is_pv_domain(d) ? __HYPERVISOR_COMPAT_VIRT_START : ~0u;
-#endif
-
     spec_ctrl_init_domain(d);
 
     if ( (rc = paging_domain_init(d)) != 0 )

xen/arch/x86/include/asm/config.h

@@ -201,7 +201,7 @@
 
 /* This is not a fixed value, just a lower limit. */
 #define __HYPERVISOR_COMPAT_VIRT_START 0xF5800000
-#define HYPERVISOR_COMPAT_VIRT_START(d) ((d)->arch.hv_compat_vstart)
+#define HYPERVISOR_COMPAT_VIRT_START(d) ((d)->arch.pv.hv_compat_vstart)
 
 #else /* !CONFIG_PV32 */
 

xen/arch/x86/include/asm/domain.h

@@ -288,6 +288,10 @@ struct pv_domain
     /* Mitigate L1TF with shadow/crashing? */
     bool check_l1tf;
 
+#ifdef CONFIG_PV32
+    unsigned int hv_compat_vstart;
+#endif
+
     /* map_domain_page() mapping cache. */
     struct mapcache_domain mapcache;
 
@@ -315,10 +319,6 @@ struct arch_domain
 {
     struct page_info *perdomain_l3_pg;
 
-#ifdef CONFIG_PV32
-    unsigned int hv_compat_vstart;
-#endif
-
     /* Maximum physical-address bitwidth supported by this guest. */
     unsigned int physaddr_bitsize;
 

xen/arch/x86/pv/dom0_build.c

@@ -521,7 +521,7 @@ static int __init dom0_construct(const struct boot_domain *bd)
                 printk("Dom0 expects too high a hypervisor start address\n");
                 return -ERANGE;
             }
-            d->arch.hv_compat_vstart =
+            d->arch.pv.hv_compat_vstart =
                 max_t(unsigned int, m2p_compat_vstart, value);
         }
 

xen/arch/x86/pv/domain.c

@@ -254,6 +254,8 @@ int switch_compat(struct domain *d)
             goto undo_and_fail;
     }
 
+    d->arch.pv.hv_compat_vstart = __HYPERVISOR_COMPAT_VIRT_START;
+
     if ( MACH2PHYS_COMPAT_NR_ENTRIES(d) < max_page )
         d->arch.physaddr_bitsize =
             /* 2^n entries can be contained in guest's p2m mapping space */