Re: [PATCH 37/74] lto, KVM: Don't assume asm statements end up in the same assembler file
From: Avi Kivity <hidden>
Date: 2012-08-19 09:01:21
Also in:
linux-kbuild, lkml
On 08/19/2012 05:56 AM, Andi Kleen wrote:
From: Andi Kleen <redacted> 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.
quoted hunk
+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