Re: [PATCH RESEND] KVM: when entering real mode align segment base to 16 bytes

[Gleb Natapov <gleb@redhat.com>]

On Mon, Dec 27, 2010 at 04:21:53PM +0200, Avi Kivity wrote:
> On 12/27/2010 03:01 PM, Gleb Natapov wrote:
> >VMX checks that base is equal segment shifted 4 bites left. Otherwise
> >guest entry fails.
> >
> >Signed-off-by: Gleb Natapov<gleb@redhat.com>
> >--
> >Same as previous one but with correct (I hope) To: header.
> >
> >diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> >index 2260783..45014ba 100644
> >--- a/arch/x86/kvm/vmx.c
> >+++ b/arch/x86/kvm/vmx.c
> >@@ -1736,7 +1736,7 @@ static void fix_rmode_seg(int seg, struct kvm_save_segment *save)
> >  	save->limit = vmcs_read32(sf->limit);
> >  	save->ar = vmcs_read32(sf->ar_bytes);
> >  	vmcs_write16(sf->selector, save->base>>  4);
> >-	vmcs_write32(sf->base, save->base&  0xfffff);
> >+	vmcs_write32(sf->base, save->base&  0xffff0);
> >  	vmcs_write32(sf->limit, 0xffff);
> >  	vmcs_write32(sf->ar_bytes, 0xf3);
> >  }
> 
> This looks okay, but I'm curious what happened here.  If the guest
> will use the segment it will fail on an incorrect address.
> 
Not much we can do about it. Either do what patch does, or exit to
userspace with more meaningful message then:

kvm: unhandled exit 80000021
kvm_run returned -22

> What's the scenario here?  And what segment is involved?  if it's fs
> or gs, or maybe ss, I can see it working out, but hardly otherwise.
> 
The scenario is older Seabios that lefts DS segment unaligned during
switch to real mode in int1587 function (copy extended memory).
 
--
			Gleb.