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Subject[PATCH 1/3] x86/xen: Avoid fast syscall path for Xen PV guests
Date
After 32-bit syscall rewrite, and specifically after commit 5f310f739b4c
("x86/entry/32: Re-implement SYSENTER using the new C path"), the stack
frame that is passed to xen_sysexit is no longer a "standard" one (i.e.
it's not pt_regs).

Since we end up calling xen_iret from xen_sysexit we don't need to fix
up the stack and instead follow entry_SYSENTER_32's IRET path directly
to xen_iret.

We can do the same thing for compat mode even though stack does not need
to be fixed. This will allow us to drop usergs_sysret32 paravirt op (in
the subsequent patch)

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Suggested-by: Andy Lutomirski <luto@amacapital.net>
---
arch/x86/entry/entry_32.S | 3 ++-
arch/x86/entry/entry_64_compat.S | 6 ++++--
arch/x86/include/asm/cpufeature.h | 1 +
arch/x86/xen/enlighten.c | 4 +++-
4 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 3eb572e..901f186 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -308,7 +308,8 @@ sysenter_past_esp:

movl %esp, %eax
call do_fast_syscall_32
- testl %eax, %eax
+ /* XEN PV guests always use IRET path */
+ ALTERNATIVE "testl %eax, %eax", "xor %eax, %eax", X86_FEATURE_XENPV
jz .Lsyscall_32_done

/* Opportunistic SYSEXIT */
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index c320183..98893d9 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -121,7 +121,8 @@ sysenter_flags_fixed:

movq %rsp, %rdi
call do_fast_syscall_32
- testl %eax, %eax
+ /* XEN PV guests always use IRET path */
+ ALTERNATIVE "testl %eax, %eax", "xor %eax, %eax", X86_FEATURE_XENPV
jz .Lsyscall_32_done
jmp sysret32_from_system_call

@@ -200,7 +201,8 @@ ENTRY(entry_SYSCALL_compat)

movq %rsp, %rdi
call do_fast_syscall_32
- testl %eax, %eax
+ /* XEN PV guests always use IRET path */
+ ALTERNATIVE "testl %eax, %eax", "xor %eax, %eax", X86_FEATURE_XENPV
jz .Lsyscall_32_done

/* Opportunistic SYSRET */
diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index e4f8010..0e4fe5b 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -216,6 +216,7 @@
#define X86_FEATURE_PAUSEFILTER ( 8*32+13) /* AMD filtered pause intercept */
#define X86_FEATURE_PFTHRESHOLD ( 8*32+14) /* AMD pause filter threshold */
#define X86_FEATURE_VMMCALL ( 8*32+15) /* Prefer vmmcall to vmcall */
+#define X86_FEATURE_XENPV ( 8*32+16) /* Xen paravirtual guest */


/* Intel-defined CPU features, CPUID level 0x00000007:0 (ebx), word 9 */
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 5774800..d315151 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1886,8 +1886,10 @@ EXPORT_SYMBOL_GPL(xen_hvm_need_lapic);

static void xen_set_cpu_features(struct cpuinfo_x86 *c)
{
- if (xen_pv_domain())
+ if (xen_pv_domain()) {
clear_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
+ set_cpu_cap(c, X86_FEATURE_XENPV);
+ }
}

const struct hypervisor_x86 x86_hyper_xen = {
--
1.8.1.4


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