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SubjectRe: [PATCH 37/74] lto, KVM: Don't assume asm statements end up in the same assembler file
On 08/19/2012 05:56 AM, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.intel.com>
>
> The VMX code references a local assembler label between two inline
> assembler statements. This assumes they both end up in the same
> assembler files. In some experimental builds of gcc this is not
> necessarily true, causing linker failures.
>
> Replace the local label reference with a more traditional asmlinkage
> extern.
>
> This also eliminates one assembler statement and
> generates a bit better code on 64bit: the compiler can
> use a RIP relative LEA instead of a movabs, saving
> a few bytes.

I'm happy to see work on lto-enabling the kernel.

>
> +extern __visible unsigned long kvm_vmx_return;
> +
> /*
> * Set up the vmcs's constant host-state fields, i.e., host-state fields that
> * will not change in the lifetime of the guest.
> @@ -3753,8 +3755,7 @@ static void vmx_set_constant_host_state(void)
> native_store_idt(&dt);
> vmcs_writel(HOST_IDTR_BASE, dt.address); /* 22.2.4 */
>
> - asm("mov $.Lkvm_vmx_return, %0" : "=r"(tmpl));
> - vmcs_writel(HOST_RIP, tmpl); /* 22.2.5 */
> + vmcs_writel(HOST_RIP, (unsigned long)&kvm_vmx_return); /* 22.2.5 */
>
> rdmsr(MSR_IA32_SYSENTER_CS, low32, high32);
> vmcs_write32(HOST_IA32_SYSENTER_CS, low32);
> @@ -6305,9 +6306,10 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
> /* Enter guest mode */
> "jne .Llaunched \n\t"
> __ex(ASM_VMX_VMLAUNCH) "\n\t"
> - "jmp .Lkvm_vmx_return \n\t"
> + "jmp kvm_vmx_return \n\t"
> ".Llaunched: " __ex(ASM_VMX_VMRESUME) "\n\t"
> - ".Lkvm_vmx_return: "
> + ".globl kvm_vmx_return\n"
> + "kvm_vmx_return: "
> /* Save guest registers, load host registers, keep flags */
> "mov %0, %c[wordsize](%%"R"sp) \n\t"
> "pop %0 \n\t"
>

The reason we use a local label is so that we the function isn't split
into two from the profiler's point of view. See cd2276a795b013d1.

One way to fix this is to have a .data variable initialized to point to
.Lkvm_vmx_return (this can be done from the same asm statement in
vmx_vcpu_run), and reference that variable in
vmx_set_constant_host_state(). If no one comes up with a better idea,
I'll write a patch doing this.

--
error compiling committee.c: too many arguments to function


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