[PATCH 6/7] x86/viridian: make the threshold for HvNotifyLongSpinWait tunable

[Paul Durrant <paul.durrant@citrix.com>]

The current threshold before the guest issues the hypercall is, and always
has been, hard-coded to 2047. It is not clear where this number came
from so, to at least allow for ease of experimentation, this patch makes
the threshold tunable via the Xen command line.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
---
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
---
 docs/misc/xen-command-line.markdown |  8 ++++++++
 xen/arch/x86/hvm/viridian.c         | 24 +++++++++++++++++++++++-
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index 4daf5b5..7f7e0d9 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1623,6 +1623,14 @@ The optional `keep` parameter causes Xen to continue using the vga
 console even after dom0 has been started.  The default behaviour is to
 relinquish control to dom0.
 
+### viridian\_spinlock\_retry\_count
+> `= <integer>`
+
+> Default: `2047`
+
+Specify the maximum number of retries before an enlightened Windows
+guest will notify Xen that it has failed to acquire a spinlock.
+
 ### vpid (Intel)
 > `= <boolean>`
 
diff --git a/xen/arch/x86/hvm/viridian.c b/xen/arch/x86/hvm/viridian.c
index deb57f9..e7cc4e4 100644
--- a/xen/arch/x86/hvm/viridian.c
+++ b/xen/arch/x86/hvm/viridian.c
@@ -22,6 +22,12 @@
 #include <public/sched.h>
 #include <public/hvm/hvm_op.h>
 
+#define VIRIDIAN_SPINLOCK_RETRY_COUNT_DEFAULT 2047
+
+static int __read_mostly viridian_spinlock_retry_count;
+integer_param("viridian_spinlock_retry_count",
+              viridian_spinlock_retry_count);
+
 /* Viridian MSR numbers. */
 #define HV_X64_MSR_GUEST_OS_ID                   0x40000000
 #define HV_X64_MSR_HYPERCALL                     0x40000001
@@ -241,7 +247,13 @@ void cpuid_viridian_leaves(const struct vcpu *v, uint32_t leaf,
             res->a |= CPUID4A_HCALL_REMOTE_TLB_FLUSH;
         if ( !cpu_has_vmx_apic_reg_virt )
             res->a |= CPUID4A_MSR_BASED_APIC;
-        res->b = 2047; /* long spin count */
+
+        /*
+         * This value is the recommended number of attempts to try to
+         * acquire a spinlock before notifying the hypervisor via the
+         * HvNotifyLongSpinWait hypercall.
+         */
+        res->b = viridian_spinlock_retry_count;
         break;
 
     case 6:
@@ -991,6 +1003,16 @@ static int viridian_load_vcpu_ctxt(struct domain *d, hvm_domain_context_t *h)
 HVM_REGISTER_SAVE_RESTORE(VIRIDIAN_VCPU, viridian_save_vcpu_ctxt,
                           viridian_load_vcpu_ctxt, 1, HVMSR_PER_VCPU);
 
+static int __init viridian_init(void)
+{
+    if ( !viridian_spinlock_retry_count )
+        viridian_spinlock_retry_count =
+            VIRIDIAN_SPINLOCK_RETRY_COUNT_DEFAULT;
+
+    return 0;
+}
+__initcall(viridian_init);
+
 /*
  * Local variables:
  * mode: C
-- 
2.1.4


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