Re: [Xen-devel] [PATCH 15/15] x86/hvm: Use system-segment relative memory accesses

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.11.16 at 16:38, <andrew.cooper3@xxxxxxxxxx> wrote:
> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -1181,20 +1181,38 @@ static int ioport_access_check(
>          return rc;
>  
>      /* Ensure the TSS has an io-bitmap-offset field. */
> -    generate_exception_if(tr.attr.fields.type != 0xb ||
> -                          tr.limit < 0x67, EXC_GP, 0);
> +    generate_exception_if(tr.attr.fields.type != 0xb, EXC_GP, 0);
>  
> -    if ( (rc = read_ulong(x86_seg_none, tr.base + 0x66,
> -                          &iobmp, 2, ctxt, ops)) )
> +    switch ( rc = read_ulong(x86_seg_tr, 0x66, &iobmp, 2, ctxt, ops) )
> +    {
> +    case X86EMUL_OKAY:
> +        break;
> +
> +    case X86EMUL_EXCEPTION:
> +        if ( !ctxt->event_pending )
> +            generate_exception_if(true, EXC_GP, 0);

generate_exception_if(!ctxt->event_pending, EXC_GP, 0) ?

> @@ -1471,9 +1490,12 @@ protmode_load_seg(
>      {
>          uint32_t new_desc_b = desc.b | a_flag;
>  
> -        if ( (rc = ops->cmpxchg(x86_seg_none, desctab.base + (sel & 0xfff8) 
> + 4,
> -                                &desc.b, &new_desc_b, 4, ctxt)) != 0 )
> +        if ( (rc = ops->cmpxchg(sel_seg, (sel & 0xfff8) + 4, &desc.b,
> +                                &new_desc_b, 4, ctxt) != X86EMUL_OKAY) )
> +        {
> +            ASSERT(rc != X86EMUL_EXCEPTION);

Hmm, now that I look at this again I don't think it's right: Why did
we think there can't be any exception here? What if the descriptor
table page is write protected? Or page tables have been changed
behind our back after the earlier read?

Jan


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