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

The value however is less obvious, and a mess.

In !PV32 builds, it's uniformly 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 uses 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>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Grygorii Strashko <grygorii_strashko@epam.com>

Texturally, but not logically, depends on "[PATCH] x86/pv: Inline
domain_set_alloc_bitsize() into it's single caller"

Bloat-o-meter reports:

  add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-65 (-65)
  Function                                     old     new   delta
  switch_compat                                472     463      -9
  arch_domain_create                          1056    1000     -56

The reason that switch_compat() gets smaller is because
MACH2PHYS_COMPAT_NR_ENTRIES(d) can now be calculated at compile time,
including the fls() thereof.
---
 xen/arch/x86/domain.c             | 5 -----
 xen/arch/x86/include/asm/config.h | 2 +-
 xen/arch/x86/include/asm/domain.h | 8 ++++----
 xen/arch/x86/pv/dom0_build.c      | 2 +-
 xen/arch/x86/pv/domain.c          | 2 ++
 5 files changed, 8 insertions(+), 11 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index d33a42c8824c..5e37bfbd17d6 100644
--- a/xen/arch/x86/domain.c
+++ b/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 )
diff --git a/xen/arch/x86/include/asm/config.h b/xen/arch/x86/include/asm/config.h
index 156369695442..cc80f2c62310 100644
--- a/xen/arch/x86/include/asm/config.h
+++ b/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 */
 
diff --git a/xen/arch/x86/include/asm/domain.h b/xen/arch/x86/include/asm/domain.h
index 386ec6174589..7e5cbd11a464 100644
--- a/xen/arch/x86/include/asm/domain.h
+++ b/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;
 
diff --git a/xen/arch/x86/pv/dom0_build.c b/xen/arch/x86/pv/dom0_build.c
index fed03dc15dcf..418b453ba8bf 100644
--- a/xen/arch/x86/pv/dom0_build.c
+++ b/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);
         }
 
diff --git a/xen/arch/x86/pv/domain.c b/xen/arch/x86/pv/domain.c
index 11db6a6d8396..ca5f6920516e 100644
--- a/xen/arch/x86/pv/domain.c
+++ b/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 */
-- 
2.39.5

On 09.12.2025 19:26, Andrew Cooper wrote:
> The hv_compat_vstart variable is hidden behind CONFIG_PV32 but lives in
> arch_domain.  Moving it into pv_domain is an obvious improvement.
> 
> The value however is less obvious, and a mess.
> 
> In !PV32 builds, it's uniformly 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 uses 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>