Re: A question about how kvm switch context to guest

[Avi Kivity <avi@redhat.com>]

On 07/03/2012 12:50 PM, Zhengwang Ruan wrote:
> Hi kashyapc & all,
> 
> I see a piece of code in vmx_vcpu_run as below, is it used to switch
> context to a guest? Kvm don't used "vmlaunch" or "vmresume" to launch or
> resume a guest?

You trimmed the bit that contains vmlaunch/vmresume.

 Why does kvm need to manually  switch context by filling
> registers using stored register copies?

Those registers don't get automatically switched by the hardware.

> 
> ===============
> 
> asm(
>         /* Store host registers */
>         "push %%"R"dx; push %%"R"bp;"
>         "push %%"R"cx \n\t" /* placeholder for guest rcx */
>         "push %%"R"cx \n\t"
>         "cmp %%"R"sp, %c[host_rsp](%0) \n\t"
>         "je 1f \n\t"
>         "mov %%"R"sp, %c[host_rsp](%0) \n\t"
>         __ex(ASM_VMX_VMWRITE_RSP_RDX) "\n\t"
>         "1: \n\t"
>         /* Reload cr2 if changed */
>         "mov %c[cr2](%0), %%"R"ax \n\t"
>         "mov %%cr2, %%"R"dx \n\t"
>         "cmp %%"R"ax, %%"R"dx \n\t"
>         "je 2f \n\t"
>         "mov %%"R"ax, %%cr2 \n\t"
>         "2: \n\t"
>         /* Check if vmlaunch of vmresume is needed */
>         "cmpl $0, %c[launched](%0) \n\t"
>         /* Load guest registers.  Don't clobber flags. */
>         "mov %c[rax](%0), %%"R"ax \n\t"
>         "mov %c[rbx](%0), %%"R"bx \n\t"
>         "mov %c[rdx](%0), %%"R"dx \n\t"
>         "mov %c[rsi](%0), %%"R"si \n\t"
>         "mov %c[rdi](%0), %%"R"di \n\t"
>         "mov %c[rbp](%0), %%"R"bp \n\t"
> 


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