linux-kernel - Re: [PATCH, RFC] x86-64: properly handle FPU code/data selectors

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Date:	Thu, 17 Oct 2013 12:41:46 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	KVM list <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Anvin <hpa@...or.com>
Subject: Re: [PATCH, RFC] x86-64: properly handle FPU code/data selectors

On Thu, Oct 17, 2013 at 10:33:33AM +0100, Jan Beulich wrote:
> >>> On 17.10.13 at 11:27, Gleb Natapov <gleb@...hat.com> wrote:
> > On Wed, Oct 16, 2013 at 05:13:40PM +0100, Jan Beulich wrote:
> >> > It preserves *less* state, because the upper 32 bits of rip are now
> >> > corrupted. Any 64-bit application that actually looks at the FP
> >> > rip/rdp fields now get the WRONG VALUES.
> >> 
> >> But again - this isn't being done for ordinary 64-bit applications,
> >> this is only happening for KVM guests. And there not being a
> >> protocol for telling the caller whether a certain context hold
> >> 64-bit offsets or selector/offset pairs shouldn't be a reason to
> >> think of a solution to the problem.
> >> 
> > KVM knows what mode guest vcpu is in. is_long_mode(vcpu) will tell you
> > if it is in long mode or not. No need to guess it.
> 
> So what if that 64-bit guest OS is running a 32-bit app? You can
> only positively know the _current_ guest word size when the
> guest is not in long mode.
> 
KVM obviously knows the complete state of virtual CPU. It can figure the
situation above by looking at CS descriptor, not need to check
is_long_mode() at all. Here is how emulator does it:

        kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);

        ctxt->eflags = kvm_get_rflags(vcpu);
        ctxt->mode = (!is_protmode(vcpu))               ?  X86EMUL_MODE_REAL :
                     (ctxt->eflags & X86_EFLAGS_VM)     ?  X86EMUL_MODE_VM86 :
                     cs_l                               ?  X86EMUL_MODE_PROT64 :
                     cs_db                              ?  X86EMUL_MODE_PROT32 :
                                                           X86EMUL_MODE_PROT16;

--
			Gleb.
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