[PATCH v3 08/15] x86: add new features for paravirt patching

[Juergen Gross <jgross@xxxxxxxx>]

For being able to switch paravirt patching from special cased custom
code sequences to ALTERNATIVE handling some X86_FEATURE_* are needed
as new features. This enables to have the standard indirect pv call
as the default code and to patch that with the non-Xen custom code
sequence via ALTERNATIVE patching later.

Make sure paravirt patching is performed before alternative patching.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
V3:
- add comment (Boris Petkov)
- no negative features (Boris Petkov)
---
 arch/x86/include/asm/cpufeatures.h |  2 ++
 arch/x86/kernel/alternative.c      | 40 ++++++++++++++++++++++++++++--
 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h 
b/arch/x86/include/asm/cpufeatures.h
index f5ef2d5b9231..1077b675a008 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -237,6 +237,8 @@
 #define X86_FEATURE_VMCALL             ( 8*32+18) /* "" Hypervisor supports 
the VMCALL instruction */
 #define X86_FEATURE_VMW_VMMCALL                ( 8*32+19) /* "" VMware prefers 
VMMCALL hypercall instruction */
 #define X86_FEATURE_SEV_ES             ( 8*32+20) /* AMD Secure Encrypted 
Virtualization - Encrypted State */
+#define X86_FEATURE_PVUNLOCK           ( 8*32+21) /* "" PV unlock function */
+#define X86_FEATURE_VCPUPREEMPT                ( 8*32+22) /* "" PV 
vcpu_is_preempted function */
 
 /* Intel-defined CPU features, CPUID level 0x00000007:0 (EBX), word 9 */
 #define X86_FEATURE_FSGSBASE           ( 9*32+ 0) /* RDFSBASE, WRFSBASE, 
RDGSBASE, WRGSBASE instructions*/
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 0a904fb2678b..abb481808811 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -600,6 +600,15 @@ int alternatives_text_reserved(void *start, void *end)
 #endif /* CONFIG_SMP */
 
 #ifdef CONFIG_PARAVIRT
+static void __init paravirt_set_cap(void)
+{
+       if (!pv_is_native_spin_unlock())
+               setup_force_cpu_cap(X86_FEATURE_PVUNLOCK);
+
+       if (!pv_is_native_vcpu_is_preempted())
+               setup_force_cpu_cap(X86_FEATURE_VCPUPREEMPT);
+}
+
 void __init_or_module apply_paravirt(struct paravirt_patch_site *start,
                                     struct paravirt_patch_site *end)
 {
@@ -623,6 +632,8 @@ void __init_or_module apply_paravirt(struct 
paravirt_patch_site *start,
 }
 extern struct paravirt_patch_site __start_parainstructions[],
        __stop_parainstructions[];
+#else
+static void __init paravirt_set_cap(void) { }
 #endif /* CONFIG_PARAVIRT */
 
 /*
@@ -730,6 +741,33 @@ void __init alternative_instructions(void)
         * patching.
         */
 
+       /*
+        * Paravirt patching and alternative patching can be combined to
+        * replace a function call with a short direct code sequence (e.g.
+        * by setting a constant return value instead of doing that in an
+        * external function).
+        * In order to make this work the following sequence is required:
+        * 1. set (artificial) features depending on used paravirt
+        *    functions which can later influence alternative patching
+        * 2. apply paravirt patching (generally replacing an indirect
+        *    function call with a direct one)
+        * 3. apply alternative patching (e.g. replacing a direct function
+        *    call with a custom code sequence)
+        * Doing paravirt patching after alternative patching would clobber
+        * the optimization of the custom code with a function call again.
+        */
+       paravirt_set_cap();
+
+       /*
+        * First patch paravirt functions, such that we overwrite the indirect
+        * call with the direct call.
+        */
+       apply_paravirt(__parainstructions, __parainstructions_end);
+
+       /*
+        * Then patch alternatives, such that those paravirt calls that are in
+        * alternatives can be overwritten by their immediate fragments.
+        */
        apply_alternatives(__alt_instructions, __alt_instructions_end);
 
 #ifdef CONFIG_SMP
@@ -748,8 +786,6 @@ void __init alternative_instructions(void)
        }
 #endif
 
-       apply_paravirt(__parainstructions, __parainstructions_end);
-
        restart_nmi();
        alternatives_patched = 1;
 }
-- 
2.26.2