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SubjectRe: [PATCH 1/6] xen/x86/pvh: Save %rbx in xen_pvh_early_cpu_init()
On 07/07/2015 04:16 PM, Konrad Rzeszutek Wilk wrote:
> On Mon, Jul 06, 2015 at 11:34:20PM -0400, Boris Ostrovsky wrote:
>> x86-64 ABI requires that functions preserve %rbx. When
>> xen_pvh_early_cpu_init() is executed on boot cpu it is invoked as a
>> function and 'cpuid' instruction will clobber %rbx. (This is not a
>> concern on secondary processors since there xen_pvh_early_cpu_init() is
>> the entry point and thus does not need to preserve registers.)
>>
>> Since we cannot access stack on secondary processors (PTE's NX bit will
>> cause #GP on AMD --- see commit a2ef5dc2c7cb ("x86/xen: Set EFER.NX and
>> EFER.SCE in PVH guests")) we can't always save %rbx on the stack. We
>> work around this by creating a new entry point for those processors ---
>> xen_pvh_early_cpu_init_secondary(). Note that on secondary CPUs we will
>> load %rbx with garbage, which is OK.
> .. as we overwrite it with %edx.

I meant here that we do 'mov -8(%rsp), %rbx' before returning and on
secondary CPUs we will grab garbage from the stack. But it's OK because
we are in the entry point and cpu_bringup_and_idle (which is where we
will be jumping to) doesn't care what's in %rbx.

Overwriting with %edx happens earlier.

-boris


>
>> As a side effect of these changes we don't need to save %rsi anymore,
>> since we can use %rbx instead of %rsi as a temporary register.
>>
>> (We could store %rbx into another scratch register such as %r8 but since
>> we will need new entry point for 32-bit PVH anyway we go with this
>> approach for symmetry)
>>
>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>
>
>> ---
>> arch/x86/xen/smp.c | 3 ++-
>> arch/x86/xen/smp.h | 4 ++++
>> arch/x86/xen/xen-head.S | 14 +++++++-------
>> 3 files changed, 13 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
>> index 8648438..ca7ee1f 100644
>> --- a/arch/x86/xen/smp.c
>> +++ b/arch/x86/xen/smp.c
>> @@ -423,7 +423,8 @@ cpu_initialize_context(unsigned int cpu, struct task_struct *idle)
>> * bit set. 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)xen_pvh_early_cpu_init;
>> + ctxt->user_regs.eip =
>> + (unsigned long)xen_pvh_early_cpu_init_secondary;
>> ctxt->user_regs.rdi = cpu;
>> ctxt->user_regs.rsi = true; /* entry == true */
>> }
>> diff --git a/arch/x86/xen/smp.h b/arch/x86/xen/smp.h
>> index 963d62a..bf2b43c 100644
>> --- a/arch/x86/xen/smp.h
>> +++ b/arch/x86/xen/smp.h
>> @@ -10,10 +10,14 @@ extern void xen_send_IPI_self(int vector);
>>
>> #ifdef CONFIG_XEN_PVH
>> extern void xen_pvh_early_cpu_init(int cpu, bool entry);
>> +extern void xen_pvh_early_cpu_init_secondary(int cpu, bool entry);
>> #else
>> static inline void xen_pvh_early_cpu_init(int cpu, bool entry)
>> {
>> }
>> +static inline void xen_pvh_early_cpu_init_secondary(int cpu, bool entry)
>> +{
>> +}
>> #endif
>>
>> #endif
>> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
>> index 8afdfcc..b1508a8 100644
>> --- a/arch/x86/xen/xen-head.S
>> +++ b/arch/x86/xen/xen-head.S
>> @@ -56,28 +56,28 @@ ENTRY(startup_xen)
>> * @entry: true if this is a secondary vcpu coming up on this entry
>> * point, false if this is the boot CPU being initialized for
>> * the first time (%rsi)
>> - *
>> - * Note: This is called as a function on the boot CPU, and is the entry point
>> - * on the secondary CPU.
>> */
>> ENTRY(xen_pvh_early_cpu_init)
>> - mov %rsi, %r11
>> + mov %rbx, -8(%rsp)
>>
>> +/* Entry point for secondary CPUs */
>> +ENTRY(xen_pvh_early_cpu_init_secondary)
>> /* Gather features to see if NX implemented. */
>> mov $0x80000001, %eax
>> cpuid
>> - mov %edx, %esi
>> + mov %edx, %ebx
>>
>> mov $MSR_EFER, %ecx
>> rdmsr
>> bts $_EFER_SCE, %eax
>>
>> - bt $20, %esi
>> + bt $20, %ebx
>> jnc 1f /* No NX, skip setting it */
>> bts $_EFER_NX, %eax
>> 1: wrmsr
>> + mov -8(%rsp), %rbx
>> #ifdef CONFIG_SMP
>> - cmp $0, %r11b
>> + cmp $0, %esi
>> jne cpu_bringup_and_idle
>> #endif
>> ret
>> --
>> 1.8.1.4
>>



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