[PATCH 1/2] x86/MCE: reduce number of MSR reads/lookups

[Jan Beulich <jbeulich@xxxxxxxx>]

MSRs mcheck_mca_logout() already reads don't need re-reading by functions
it calls, even less so when it calls them exclusively. Seeing reports like
[1], this also makes sure that what we use internally also is propagated
to the MCE log.

While touching function signatures, also switch bank number parameters to
unsigned int.

[1] https://lists.xen.org/archives/html/xen-devel/2026-10/msg01825.html

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
mcheck_cmn_handler()'s read of MSR_IA32_MCG_STATUS could likely also be
dropped, by having mcheck_mca_logout() provide the value it read.

--- a/xen/arch/x86/cpu/mcheck/mce.c
+++ b/xen/arch/x86/cpu/mcheck/mce.c
@@ -138,12 +138,8 @@ void mcabanks_free(struct mca_banks *ban
     xfree(banks);
 }
 
-static void mcabank_clear(int banknum)
+static void mcabank_clear(unsigned int banknum, uint64_t status)
 {
-    uint64_t status;
-
-    status = mca_rdmsr(MSR_IA32_MCx_STATUS(banknum));
-
     if ( status & MCi_STATUS_ADDRV )
         mca_wrmsr(MSR_IA32_MCx_ADDR(banknum), 0x0ULL);
     if ( status & MCi_STATUS_MISCV )
@@ -165,7 +161,8 @@ const struct mca_error_handler *__read_m
 unsigned int __read_mostly mce_dhandler_num;
 unsigned int __read_mostly mce_uhandler_num;
 
-static void mca_init_bank(enum mca_source who, struct mc_info *mi, int bank)
+static void mca_init_bank(enum mca_source who, struct mc_info *mi,
+                          unsigned int bank, uint64_t status)
 {
     struct mcinfo_bank *mib;
 
@@ -179,20 +176,20 @@ static void mca_init_bank(enum mca_sourc
         return;
     }
 
-    mib->mc_status = mca_rdmsr(MSR_IA32_MCx_STATUS(bank));
+    mib->mc_status = status;
 
     mib->mc_bank = bank;
     mib->mc_domid = DOMID_INVALID;
 
-    if ( mib->mc_status & MCi_STATUS_MISCV )
+    if ( status & MCi_STATUS_MISCV )
         mib->mc_misc = mca_rdmsr(MSR_IA32_MCx_MISC(bank));
 
-    if ( mib->mc_status & MCi_STATUS_ADDRV )
+    if ( status & MCi_STATUS_ADDRV )
         mib->mc_addr = mca_rdmsr(MSR_IA32_MCx_ADDR(bank));
 
-    if ( (mib->mc_status & MCi_STATUS_MISCV) &&
-         (mib->mc_status & MCi_STATUS_ADDRV) &&
-         alternative_call(mce_callbacks.check_addr, mib->mc_status,
+    if ( (status & MCi_STATUS_MISCV) &&
+         (status & MCi_STATUS_ADDRV) &&
+         alternative_call(mce_callbacks.check_addr, status,
                           mib->mc_misc, MC_ADDR_PHYSICAL) &&
          (who == MCA_POLLER || who == MCA_CMCI_HANDLER) &&
          (mfn_valid(_mfn(paddr_to_pfn(mib->mc_addr)))) )
@@ -211,14 +208,13 @@ static void mca_init_bank(enum mca_sourc
     }
 }
 
-static int mca_init_global(uint32_t flags, struct mcinfo_global *mig)
+static int mca_init_global(uint32_t flags, struct mcinfo_global *mig,
+                           uint64_t status)
 {
-    uint64_t status;
     int cpu_nr;
     const struct vcpu *curr = current;
 
     /* Set global information */
-    status = mca_rdmsr(MSR_IA32_MCG_STATUS);
     mig->mc_gstatus = status;
     mig->mc_domid = DOMID_INVALID;
     mig->mc_vcpuid = XEN_MC_VCPUID_INVALID;
@@ -331,7 +327,7 @@ mcheck_mca_logout(enum mca_source who, s
                 mig = x86_mcinfo_reserve(mci, sizeof(*mig), MC_TYPE_GLOBAL);
                 /* mc_info should at least hold up the global information */
                 ASSERT(mig);
-                mca_init_global(mc_flags, mig);
+                mca_init_global(mc_flags, mig, gstatus);
                 /* A hook here to get global extended msrs */
                 if ( IS_ENABLED(CONFIG_INTEL) &&
                      boot_cpu_data.x86_vendor == X86_VENDOR_INTEL )
@@ -351,7 +347,7 @@ mcheck_mca_logout(enum mca_source who, s
             /* uc = true, recover = true, we need not panic. */
             recover = alternative_call(mce_callbacks.recoverable_scan, status);
 
-        mca_init_bank(who, mci, i);
+        mca_init_bank(who, mci, i, status);
 
         if ( mce_callbacks.info_collect )
             alternative_vcall(mce_callbacks.info_collect, mci, i, status);
@@ -359,7 +355,7 @@ mcheck_mca_logout(enum mca_source who, s
         /* By default, need_clear = true */
         if ( who != MCA_MCE_SCAN && need_clear )
             /* Clear bank */
-            mcabank_clear(i);
+            mcabank_clear(i, status);
         else if ( who == MCA_MCE_SCAN && need_clear )
             mcabanks_set(i, clear_bank);
     }
@@ -548,7 +544,7 @@ void mcheck_mca_clearbanks(struct mca_ba
     {
         if ( !mcabanks_test(i, bankmask) )
             continue;
-        mcabank_clear(i);
+        mcabank_clear(i, mca_rdmsr(MSR_IA32_MCx_STATUS(i)));
     }
 }