[Xen-devel] [V1 PATCH 2/2] PVH: set EFER.NX and EFER.SCE for secondary vcpus

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

This patch addresses three things for a pvh secondary vcpu:

  - NX bug on intel: It was recenlty discovered that NX is not being
    honored in PVH on intel since EFER.NX is not being set. The pte.NX
    bits are ignored if EFER.NX is not set on intel.

  - PVH boot hang on newer xen:  Following c/s on xen

            c/s 7645640:  x86/PVH: don't set EFER_SCE for pvh guest

    removes setting of EFER.SCE for PVH guests. As such, existing intel pvh
    guest will no longer boot on xen after that c/s.

  - Both above changes will be applicable to AMD also when xen support of
    AMD pvh is added.

Please note: We create a new glue assembly entry point because the
secondary vcpus come up on kernel page tables that have pte.NX
bits set. While on Intel these are ignored if EFER.NX is not set, on
AMD a RSVD bit fault is generated.

Signed-off-by: Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>
---
 arch/x86/xen/smp.c      | 28 ++++++++++++++++++++--------
 arch/x86/xen/smp.h      |  1 +
 arch/x86/xen/xen-head.S | 21 +++++++++++++++++++++
 3 files changed, 42 insertions(+), 8 deletions(-)

diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
index 7005974..66058b9 100644
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -37,6 +37,7 @@
 #include <xen/hvc-console.h>
 #include "xen-ops.h"
 #include "mmu.h"
+#include "smp.h"
 
 cpumask_var_t xen_cpu_initialized_map;
 
@@ -99,8 +100,12 @@ static void cpu_bringup(void)
        wmb();                  /* make sure everything is out */
 }
 
-/* Note: cpu parameter is only relevant for PVH */
-static void cpu_bringup_and_idle(int cpu)
+/*
+ * Note: cpu parameter is only relevant for PVH. The reason for passing it
+ * is we can't do smp_processor_id until the percpu segments are loaded, for
+ * which we need the cpu number! So we pass it in rdi as first parameter.
+ */
+asmlinkage __visible void cpu_bringup_and_idle(int cpu)
 {
 #ifdef CONFIG_X86_64
        if (xen_feature(XENFEAT_auto_translated_physmap) &&
@@ -374,11 +379,10 @@ cpu_initialize_context(unsigned int cpu, struct 
task_struct *idle)
        ctxt->user_regs.fs = __KERNEL_PERCPU;
        ctxt->user_regs.gs = __KERNEL_STACK_CANARY;
 #endif
-       ctxt->user_regs.eip = (unsigned long)cpu_bringup_and_idle;
-
        memset(&ctxt->fpu_ctxt, 0, sizeof(ctxt->fpu_ctxt));
 
        if (!xen_feature(XENFEAT_auto_translated_physmap)) {
+               ctxt->user_regs.eip = (unsigned long)cpu_bringup_and_idle;
                ctxt->flags = VGCF_IN_KERNEL;
                ctxt->user_regs.eflags = 0x1000; /* IOPL_RING1 */
                ctxt->user_regs.ds = __USER_DS;
@@ -416,12 +420,20 @@ cpu_initialize_context(unsigned int cpu, struct 
task_struct *idle)
 #ifdef CONFIG_X86_32
        }
 #else
-       } else
-               /* N.B. The user_regs.eip (cpu_bringup_and_idle) is called with
-                * %rdi having the cpu number - which means are passing in
-                * as the first parameter the cpu. Subtle!
+       } else {
+               /*
+                * The vcpu comes on kernel page tables which have the NX pte
+                * bit set on AMD. This means before DS/SS is touched, NX in
+                * EFER must be set. Hence the following assembly glue code.
+                */
+               ctxt->user_regs.eip = (unsigned long)pvh_cpu_bringup;
+
+               /* N.B. The bringup function cpu_bringup_and_idle is called with
+                * %rdi having the cpu number - which means we are passing it in
+                * as the first parameter. Subtle!
                 */
                ctxt->user_regs.rdi = cpu;
+       }
 #endif
        ctxt->user_regs.esp = idle->thread.sp0 - sizeof(struct pt_regs);
        ctxt->ctrlreg[3] = xen_pfn_to_cr3(virt_to_mfn(swapper_pg_dir));
diff --git a/arch/x86/xen/smp.h b/arch/x86/xen/smp.h
index c7c2d89..b20ba68 100644
--- a/arch/x86/xen/smp.h
+++ b/arch/x86/xen/smp.h
@@ -7,5 +7,6 @@ extern void xen_send_IPI_mask_allbutself(const struct cpumask 
*mask,
 extern void xen_send_IPI_allbutself(int vector);
 extern void xen_send_IPI_all(int vector);
 extern void xen_send_IPI_self(int vector);
+extern void pvh_cpu_bringup(int cpu);
 
 #endif
diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 485b695..db8dca5 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -47,6 +47,27 @@ ENTRY(startup_xen)
 
        __FINIT
 
+#ifdef CONFIG_XEN_PVH
+#ifdef CONFIG_X86_64
+/* Note that rdi contains the cpu number and must be preserved */
+ENTRY(pvh_cpu_bringup)
+       /* Gather features to see if NX implemented. (no EFER.NX on intel) */
+       movl    $0x80000001, %eax
+       cpuid
+       movl    %edx,%esi
+
+       movl    $MSR_EFER, %ecx
+       rdmsr
+       btsl    $_EFER_SCE, %eax
+
+       btl     $20,%esi
+       jnc     1f      /* No NX, skip it */
+       btsl    $_EFER_NX, %eax
+1:     wrmsr
+       jmp cpu_bringup_and_idle
+#endif /* CONFIG_X86_64 */
+#endif /* CONFIG_XEN_PVH */
+
 .pushsection .text
        .balign PAGE_SIZE
 ENTRY(hypercall_page)
-- 
1.8.3.1


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