Paolo Bonzini <pbonz...@redhat.com> 于2019年9月26日周四 下午5:53写道：
> On 26/09/19 11:35, Li Qiang wrote: > > So without unrestrict guest the mainline is this: KVM set guest's > > rflag bit X86_EFLAGS_VM, so when the guest enter guest mode, it is in > > vm86 mode. In this mode, the CPU will access the address like in > > real mode(seg*4+offset), this address is linear address. And in fact, > > the vm86 is still in protected, so the linear address will be > > translated to gpa by the identity mapping table. Then goes to EPT > > table? > > Yes. > > > ... as soon as the guest tries to enter protected mode, it will get > into > > a situation which is not real mode but doesn't have the segment > > registers properly loaded with selectors. > > > > Therefore, it will either > > hack things together (enter_pmode) or emulate instructions until the > > state is accepted even without unrestricted guest support. > > > > Could you please explain this situation more detailed? Why this happen? > > Protected mode entry looks like this: > > mov %cr0, %eax > or $1, %al > mov %eax, %cr0 > # [1] now in 16-bit protected mode > lgdtl gdt32 > ljmpl $8, 2f > # [2] now in 32-bit protected mode > 2: > .code32 > mov $16, %ax > mov %ax, %ds > mov %ax, %es > mov %ax, %fs > mov %ax, %gs > mov %ax, %ss > # [3] now everything is okay > > Between [1] and [3] the vmentry could fail if not in unrestricted mode. > For example (see checks on guest segment registers in the SDM): > > - "CS. Type must be 9, 11, 13, or 15 (accessed code segment)." CS in > real-mode is a RW data segment, not a code segment. This applies > between [1] and [2]. > > - "SS. If the guest will not be virtual-8086 and the “unrestricted > guest” VM-execution control is 0, the RPL (bits 1:0) must equal the RPL > of the selector field for CS." This may not be the case if the segment > register still holds real-mode values (which are not selectors, just > base >> 4). This applies between [1] and [3]. > > - "DS, ES, FS, GS. The DPL cannot be less than the RPL in the selector > field" Again, the real-mode DPL is zero but the RPL makes no sense if > the segment registers hold a real-mode value. > > You can find more about these checks in guest_state_valid(); look at the > "else" branch of that function, the "then" branch is for pmode->rmode > transitions. When any of the checks fail, KVM emulates instructions > instead of using VMX non-root mode (usually it's just a handful of them, > as in the case above). > > Thanks so much for your explanation. I will read the code more to strengthen my understanding. Thanks, Li Qiang > Paolo > >
