A question about how kvm switch context to guest

[Zhengwang Ruan <ruan.zhengwang@gmail.com>]

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

===============

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"

========

Thanks,

Zhengwang