Re: [Xen-devel] [V5] x86/xsaves: fix overwriting between non-lazy/lazy xsaves

[Shuai Ruan <shuai.ruan@xxxxxxxxxxxxxxx>]

On Wed, Mar 23, 2016 at 10:02:24AM +0800, Shuai Ruan wrote:
> > > -    /* Set XSTATE_BV and XCOMP_BV.  */
> > > +    /* Set XSTATE_BV.  */
> > >      xsave->xsave_hdr.xstate_bv = xstate_bv;
> > > -    xsave->xsave_hdr.xcomp_bv = v->arch.xcr0_accum | 
> > > XSTATE_COMPACTION_ENABLED;
> > >      setup_xstate_comp(xstate_comp_offsets, xstate_bv);
> > 
> > I see you set xcomp_bv (and hence the compaction bit) in xrstor()
> > now, but afaict that doesn't allow you to completely drop initializing
> > the field here, as the code there looks at the compaction bit.
> > 
> > > +                if ( unlikely(!(ptr->xsave_hdr.xcomp_bv &
> > > +                                XSTATE_COMPACTION_ENABLED)) )
> > > +                ptr->xsave_hdr.xcomp_bv = ptr->xsave_hdr.xstate_bv
> > > +                                           |XSTATE_COMPACTION_ENABLED;
> > > +
> > > +                XRSTOR("0x48,","0x0f,0xc7,0x1f"); /* xrstors */
> > > +             }
> > > +            else
> > > +                XRSTOR("0x48,","0x0f,0xae,0x2f"); /* xrstor */
> > 
> > At this point, what guarantees that xcomp_bv is zero, no matter
> > where the state to be loaded originates from? I think at least in
> > arch_set_info_guest(), hvm_load_cpu_ctxt(), and
> > hvm_vcpu_reset_state() you went too far deleting code, and you
> > really need to keep the storing of zero there. Did you draw, just
> > for yourself, mentally or on a sheet of paper, a diagram illustrating
> > the various state transitions?
> > 
> For above two comments.
> 
> The patch is base on [v4]x86/xsaves: calculate the xstate_comp_offsets base
> on xstate_bv and I answer your question on why caculate xstate_comp_offset
> based on xstate_bv in previous thread. If that is right, drop
> initializing xcomp_bv is ok. Now xcomp_bv can guarantee to be zero for 
> arch_set_info_guest() and hvm_load_cpu_ctxt(). If the drop is wrong
> (due to my misunderstand of the SDM), I will change the if () here.
> 
Ignore the above paragraph, I realized that [v4]x86/xsaves: calculate
the xstate_comp_offsets base on xstate_bv is wrong(should be base on
xcomp_bv as previous version). Then I should not delete the code 
initializing the xcomp_bv in compress_xsave_state() 
But for hvm_vcpu_reset_state(), I think we should deleting the code
initializing the xcomp_bv as said below.
> For hvm_vcpu_reset_state(), we should depend on whether xsaves is used 
> to decide whether to init xcomp_bv or not. And currently we use
> xcr0_accum to indicate the use of xsaves, when hvm_vcpu_reset_state()
> is called , can vcpu->xcr0_accum indicate using of xsaves ?
> I think in hvm_vcpu_reset_state(), we should leave xcomp_bv zero.
> 

> > Since again you repeat the same logic twice, this should again have
> > been a signal that all your changes should go into the XRSTOR()
> > macro. Or alternatively, since the exception fixup also differs, you
> > may want to convert the whole logic into an XSAVES and an XSAVE
> > path. My only really sincere request here is - as little redundancy as
> > possible, since having to change the same thing twice in more than
> > one place is always calling for trouble.
I will do all changes only in XRSTOR(). Code like : 

#define _XRSTOR(pfx, xrstor_ins)
       asm volatile ( "1: .byte " pfx xrstor_ins"\n" \
                      "3:\n" \
                      "   .section .fixup,\"ax\"\n" \
                      "2: incl %[faults]\n" \
                      "   jmp 3b\n" \
                      "   .previous\n" \
                      _ASM_EXTABLE(1b, 2b) \
                      : [mem] "+m" (*ptr), [faults] "+g" (faults) \
                      : [lmask] "a" (lmask), [hmask] "d" (hmask), \
                      [ptr] "D" (ptr) )

#define XRSTOR(pfx) \
            if ( v->arch.xcr0_accum & XSTATE_XSAVES_ONLY ) \
            { \
                if ( unlikely(!(ptr->xsave_hdr.xcomp_bv \
                                & XSTATE_COMPACTION_ENABLED)) ) \
                    ptr->xsave_hdr.xcomp_bv |= (ptr->xsave_hdr.xstate_bv \
                                                | XSTATE_COMPACTION_ENABLED); \
                _XRSTOR("0x48, ", "0x0f,0xc7,0x1f"); \
            } \
            else \
            { \
                _XRSTOR("0x48, ", "0x0f,0xae,0x2f"); \
            }
....
#undef XRSTOR
#undef _XRSTOR

A now wapper is intruduced as "_XRSTOR"( maybe the macro name is not
good ). 

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