[xen stable-4.17] x86/cpu-policy: Fix migration from Ice Lake to Cascade Lake

[patchbot@xxxxxxx]

commit c5c46464704d4b96a57bcbf4718e092d6233d913
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue May 21 11:59:09 2024 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue May 21 11:59:09 2024 +0200

    x86/cpu-policy: Fix migration from Ice Lake to Cascade Lake
    
    Ever since Xen 4.14, there has been a latent bug with migration.
    
    While some toolstacks can level the features properly, they don't shink
    feat.max_subleaf when all features have been dropped.  This is because
    we *still* have not completed the toolstack side work for full CPU Policy
    objects.
    
    As a consequence, even when properly feature levelled, VMs can't migrate
    "backwards" across hardware which reduces feat.max_subleaf.  One such 
example
    is Ice Lake (max_subleaf=2 for INTEL_PSFD) to Cascade Lake (max_subleaf=0).
    
    Extend the max policies feat.max_subleaf to the hightest number Xen knows
    about, but leave the default policies matching the host.  This will allow 
VMs
    with a higher feat.max_subleaf than strictly necessary to migrate in.
    
    Eventually we'll manage to teach the toolstack how to avoid creating such 
VMs
    in the first place, but there's still more work to do there.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    master commit: a2330b51df267e20e66bbba6c5bf08f0570ed58b
    master date: 2024-05-07 16:56:46 +0100
---
 xen/arch/x86/cpu-policy.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c
index e44de3cfcb..c813df35cb 100644
--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -596,6 +596,13 @@ static void __init calculate_pv_max_policy(void)
     unsigned int i;
 
     *p = host_cpu_policy;
+
+    /*
+     * Some VMs may have a larger-than-necessary feat max_subleaf.  Allow them
+     * to migrate in.
+     */
+    p->feat.max_subleaf = ARRAY_SIZE(p->feat.raw) - 1;
+
     x86_cpu_policy_to_featureset(p, fs);
 
     for ( i = 0; i < ARRAY_SIZE(fs); ++i )
@@ -636,6 +643,10 @@ static void __init calculate_pv_def_policy(void)
     unsigned int i;
 
     *p = pv_max_cpu_policy;
+
+    /* Default to the same max_subleaf as the host. */
+    p->feat.max_subleaf = host_cpu_policy.feat.max_subleaf;
+
     x86_cpu_policy_to_featureset(p, fs);
 
     for ( i = 0; i < ARRAY_SIZE(fs); ++i )
@@ -672,6 +683,13 @@ static void __init calculate_hvm_max_policy(void)
     const uint32_t *mask;
 
     *p = host_cpu_policy;
+
+    /*
+     * Some VMs may have a larger-than-necessary feat max_subleaf.  Allow them
+     * to migrate in.
+     */
+    p->feat.max_subleaf = ARRAY_SIZE(p->feat.raw) - 1;
+
     x86_cpu_policy_to_featureset(p, fs);
 
     mask = hvm_hap_supported() ?
@@ -758,6 +776,10 @@ static void __init calculate_hvm_def_policy(void)
     const uint32_t *mask;
 
     *p = hvm_max_cpu_policy;
+
+    /* Default to the same max_subleaf as the host. */
+    p->feat.max_subleaf = host_cpu_policy.feat.max_subleaf;
+
     x86_cpu_policy_to_featureset(p, fs);
 
     mask = hvm_hap_supported() ?
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.17