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

Juergen Gross
2020-12-17T09:34:06Z

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@suse.com>
---
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