[PATCH v4 2/3] 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>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
Cc: Andrew Cooper <andrew.cooper3@citrix.com>

v3:
 - Use '-' in parameter name rather than '_'

v2:
 - Significantly simplify patch by directly initializing the parameter
---
 docs/misc/xen-command-line.markdown |  8 ++++++++
 xen/arch/x86/hvm/viridian.c         | 16 +++++++++++++++-
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index c81d693..bdbdb8a 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1624,6 +1624,14 @@ relinquish control to dom0.
 <major>, <minor> and <build> must be integers. The values will be
 encoded in guest CPUID 0x40000002 if viridian enlightenments are enabled.
 
+### 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 3d58416..e18a453 100644
--- a/xen/arch/x86/hvm/viridian.c
+++ b/xen/arch/x86/hvm/viridian.c
@@ -174,6 +174,14 @@ static uint16_t viridian_major = 6;
 static uint16_t viridian_minor = 0;
 static uint32_t viridian_build = 0x1772;
 
+/*
+ * Maximum number of retries before the guest will notify of failure
+ * to acquire a spinlock.
+ */
+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)
 {
@@ -251,7 +259,13 @@ 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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