[PATCH v2] x86/SVM: restrict hardware SSBD update upon guest VIRT_SPEC_CTRL write

[Jan Beulich <jbeulich@xxxxxxxx>]

core_set_legacy_ssbd() counts the number of times SSBD is being enabled
via LS_CFG on a core. This assumes that calls there only occur if the
state actually changes. While svm_ctxt_switch_{to,from}() conform to
this, guest_wrmsr() doesn't: It also calls the function when the bit
doesn't actually change. Make core_set_legacy_ssbd() track per-thread
enabled state by converting the "count" field to a bitmap, thus allowing
to skip redundant enable/disable requests, constraining
amd_setup_legacy_ssbd() accordingly.

Fixes: b2030e6730a2 ("amd/virt_ssbd: set SSBD at vCPU context switch")
Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
This wants properly testing on affected hardware. From Andrew's
description it's also not clear whether this really is addressing that
problem, or yet another one in this same area.
---
v2: Change core_set_legacy_ssbd() itself rather than the problematic
    caller.

--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -744,7 +744,7 @@ void amd_init_ssbd(const struct cpuinfo_
 
 static struct ssbd_ls_cfg {
     spinlock_t lock;
-    unsigned int count;
+    unsigned long enabled;
 } __cacheline_aligned *ssbd_ls_cfg;
 static unsigned int __ro_after_init ssbd_max_cores;
 #define AMD_FAM17H_MAX_SOCKETS 2
@@ -757,6 +757,11 @@ bool __init amd_setup_legacy_ssbd(void)
            boot_cpu_data.x86_num_siblings <= 1 || opt_ssbd)
                return true;
 
+       if (boot_cpu_data.x86_num_siblings > BITS_PER_LONG ||
+           (boot_cpu_data.x86_num_siblings &
+            (boot_cpu_data.x86_num_siblings - 1)))
+               return false;
+
        /*
         * One could be forgiven for thinking that c->x86_max_cores is the
         * correct value to use here.
@@ -800,10 +805,12 @@ static void core_set_legacy_ssbd(bool en
                              c->cpu_core_id];
 
        spin_lock_irqsave(&status->lock, flags);
-       status->count += enable ? 1 : -1;
-       ASSERT(status->count <= c->x86_num_siblings);
-       if (enable ? status->count == 1 : !status->count)
+       if (!enable)
+               __clear_bit(c->apicid & (c->x86_num_siblings - 1), 
&status->enabled);
+       if (!status->enabled)
                BUG_ON(!set_legacy_ssbd(c, enable));
+       if (enable)
+               __set_bit(c->apicid & (c->x86_num_siblings - 1), 
&status->enabled);
        spin_unlock_irqrestore(&status->lock, flags);
 }