[PATCH v2 5/6] 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>

v2:
 - Significantly simplify patch by directly initializing the parameter
---
 docs/misc/xen-command-line.markdown |  8 ++++++++
 xen/arch/x86/hvm/viridian.c         | 12 +++++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index 44da4bd..962e4d1 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1637,6 +1637,14 @@ The hypervisor minor version number encoded in CPUID 0x40000002:EBX.
 
 The hypervisor build number encoded in CPUID 0x40000002:EAX.
 
+### 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 d241de2..5ca1167 100644
--- a/xen/arch/x86/hvm/viridian.c
+++ b/xen/arch/x86/hvm/viridian.c
@@ -179,6 +179,10 @@ integer_param("viridian_minor_version", viridian_minor_version);
 static uint32_t __read_mostly viridian_build_number = 0x1772;
 integer_param("viridian_build_number", viridian_build_number);
 
+static uint32_t __read_mostly viridian_spinlock_retry_count = 2047;
+integer_param("viridian_spinlock_retry_count",
+              viridian_spinlock_retry_count);
+
 void cpuid_viridian_leaves(const struct vcpu *v, uint32_t leaf,
                            uint32_t subleaf, struct cpuid_leaf *res)
 {
@@ -257,7 +261,13 @@ 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:
-- 
2.1.4


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