[Xen-devel] [PATCH 6/9] x86/amd: Allocate resources to cope with LS_CFG being per-core on Fam17h

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The need for per-core resources is a property of Fam17h hardware.  The
mechanim for calculating / allocating space is all a bit horrible, but is the
best which can be done at this point.  See the code comments for details.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Parts based on an earlier patch by Brian
Signed-off-by: Signed-off-by: Brian Woods <brian.woods@xxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Brian Woods <brian.woods@xxxxxxx>
---
 xen/arch/x86/cpu/amd.c | 91 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 91 insertions(+)

diff --git a/xen/arch/x86/cpu/amd.c b/xen/arch/x86/cpu/amd.c
index 897c060..ea10dbd 100644
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -419,6 +419,97 @@ static void __init noinline amd_probe_legacy_ssbd(void)
 }
 
 /*
+ * This is all a gross hack, but Xen really doesn't have flexible-enough
+ * per-cpu infrastructure to do it properly.  For Zen(v1) with SMT active,
+ * MSR_AMD64_LS_CFG is per-core rather than per-thread, so we need a per-core
+ * spinlock to synchronise updates of the MSR.
+ *
+ * We can't use per-cpu state because taking one CPU offline would free state
+ * under the feet of another.  Ideally, we'd allocate memory on the AP boot
+ * path, but by the time the sibling information is calculated sufficiently
+ * for us to locate the per-core state, it's too late to fail the AP boot.
+ *
+ * We also can't afford to end up in a heterogeneous scenario with some CPUs
+ * unable to safely use LS_CFG.
+ *
+ * Therefore, we have to allocate for the worse-case scenario, which is
+ * believed to be 4 sockets.  Any allocation failure cause us to turn LS_CFG
+ * off, as this is fractionally better than failing to boot.
+ */
+static struct ssbd_ls_cfg {
+       spinlock_t lock;
+       unsigned int disable_count;
+} *ssbd_ls_cfg[4];
+static unsigned int ssbd_max_cores;
+
+static int __init amd_init_legacy_ssbd(void)
+{
+       const struct cpuinfo_x86 *c = &boot_cpu_data;
+       unsigned int socket, core;
+
+       /* No legacy SSBD interface?  Nothing to do. */
+       if (!cpu_has_legacy_ssbd)
+               return 0;
+
+       /*
+        * No setup required for:
+        *  - Using MSR_VIRT_SPEC_CTRL
+        *  - Pre-Fam17h hardware
+        *  - Fam17h with SMT disabled
+        */
+       if (cpu_has_virt_sc_ssbd || c->x86 < 0x17 || c->x86_num_siblings == 1)
+               return 0;
+
+       /*
+        * One could be forgiven for thinking that c->x86_max_cores is the
+        * correct value to use here.
+        *
+        * However, that value is derived from the current configuration, and
+        * c->cpu_core_id is sparse on all but the top end CPUs.  Derive
+        * max_cpus from ApicIdCoreIdSize which will cover any sparseness.
+        */
+       if (c->extended_cpuid_level >= 0x80000008) {
+               ssbd_max_cores = 1u << MASK_EXTR(cpuid_ecx(0x80000008), 0xf000);
+               ssbd_max_cores /= c->x86_num_siblings;
+       }
+
+       if (ssbd_max_cores == 0) {
+               printk(XENLOG_WARNING
+                      "Failed to calculate max_cores. Disabling 
LEGACY_SSBD\n");
+               setup_clear_cpu_cap(X86_FEATURE_LEGACY_SSBD);
+
+               return 0;
+       }
+
+       /*
+        * Allocate the worse case cores-per-socket worth of state, as
+        * described by hardware.
+        */
+       for (socket = 0; socket < ARRAY_SIZE(ssbd_ls_cfg); ++socket) {
+               ssbd_ls_cfg[socket] =
+                       xzalloc_array(struct ssbd_ls_cfg, ssbd_max_cores);
+
+               if (!ssbd_ls_cfg[socket]) {
+                       /*
+                        * Memory allocation failure.  Backtrack and pretend
+                        * that the LEGACY_SSBD interface isn't available.
+                        */
+                       printk(XENLOG_WARNING
+                              "Out of memory.  Disabling LEGACY_SSBD\n");
+                       setup_clear_cpu_cap(X86_FEATURE_LEGACY_SSBD);
+
+                       return 0;
+               }
+
+               for (core = 0; core < ssbd_max_cores; ++core)
+                       spin_lock_init(&ssbd_ls_cfg[socket][core].lock);
+       }
+
+       return 0;
+}
+presmp_initcall(amd_init_legacy_ssbd);
+
+/*
  * Check for the presence of an AMD erratum. Arguments are defined in amd.h 
  * for each known erratum. Return 1 if erratum is found.
  */
-- 
2.1.4


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