Re: [PATCH/RFC] Let gcc to choose which registers to save

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Laurent Vivier wrote:
> As x86_64 ABI defines some registers saved by the calling function, it is not
> needed to save all registers in the called function when switching to VCPU.
> (see http://www.x86-64.org/documentation/abi.pdf, chapter 3.2.1)
>
> The best way to do that is to inform GCC which registers we use and let
> it to save only needed registers.
>
>   

Strange, yesterday I started to do the same thing but dropped it after I 
got discouraged by reload errors from gcc.
> diff --git a/drivers/kvm/vmx.c b/drivers/kvm/vmx.c
> index 2c6b64a..d6c91ac 100644
> --- a/drivers/kvm/vmx.c
> +++ b/drivers/kvm/vmx.c
> @@ -2243,16 +2243,12 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
>  	asm(
>  		/* Store host registers */
>  #ifdef CONFIG_X86_64
> -		"push %%rax; push %%rbx; push %%rdx;"
> -		"push %%rsi; push %%rdi; push %%rbp;"
> -		"push %%r8;  push %%r9;  push %%r10; push %%r11;"
> -		"push %%r12; push %%r13; push %%r14; push %%r15;"
> +		"push %%rdx; push %%rbp;"
>  		"push %%rcx \n\t"
> -		ASM_VMX_VMWRITE_RSP_RDX "\n\t"
>  #else
>  		"pusha; push %%ecx \n\t"
> -		ASM_VMX_VMWRITE_RSP_RDX "\n\t"
>  #endif
> +		ASM_VMX_VMWRITE_RSP_RDX "\n\t"
>  		/* Check if vmlaunch of vmresume is needed */
>  		"cmp $0, %1 \n\t"
>  		/* Load guest registers.  Don't clobber flags. */
> @@ -2311,12 +2307,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
>  		"mov %%r15, %c[r15](%3) \n\t"
>  		"mov %%cr2, %%rax   \n\t"
>  		"mov %%rax, %c[cr2](%3) \n\t"
> -		"mov (%%rsp), %3 \n\t"
>  
> -		"pop  %%rcx; pop  %%r15; pop  %%r14; pop  %%r13; pop  %%r12;"
> -		"pop  %%r11; pop  %%r10; pop  %%r9;  pop  %%r8;"
> -		"pop  %%rbp; pop  %%rdi; pop  %%rsi;"
> -		"pop  %%rdx; pop  %%rbx; pop  %%rax \n\t"
> +		"pop  %%rcx; pop  %%rbp; pop  %%rdx \n\t"
>  #else
>  		"xchg %3, (%%esp) \n\t"
>  		"mov %%eax, %c[rax](%3) \n\t"
> @@ -2354,7 +2346,12 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
>  		[r15]"i"(offsetof(struct kvm_vcpu, regs[VCPU_REGS_R15])),
>  #endif
>  		[cr2]"i"(offsetof(struct kvm_vcpu, cr2))
> -	      : "cc", "memory");
> +	      : "cc", "memory",
> +#ifdef CONFIG_X86_64
> +		"rbx", "rdi", "rsi",
> +		"r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15"
> +#endif
> +	      );
>  

The comma after "memory" worries me.  Can you compile-test on i386?

Other than that the patch is very welcome -- the excessive register 
saving is very annoying to me.

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


-------------------------------------------------------------------------
This SF.net email is sponsored by: Splunk Inc.
Still grepping through log files to find problems?  Stop.
Now Search log events and configuration files using AJAX and a browser.
Download your FREE copy of Splunk now >> http://get.splunk.com/