Re: [Xen-devel] [PATCH v3 4/4] KVM: VMX: Simplify segment_base

[Jim Mattson <jmattson@xxxxxxxxxx>]

Can we use the read-only GDT here? When expanding the virtual address
for 64-bit system descriptors, isn't it sufficient to check (d->s == 0
&& d->type != 0)?

On Tue, Feb 14, 2017 at 11:42 AM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
> The KVM segment_base function is confusing. This patch replaces integers
> with appropriate flags, simplify constructs and add comments.
>
> Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
> ---
> Based on next-20170213
> ---
>  arch/x86/kvm/vmx.c | 26 ++++++++++++++++++--------
>  1 file changed, 18 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 99167f20bc34..edb8326108dd 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2062,25 +2062,35 @@ static unsigned long segment_base(u16 selector)
>         struct desc_struct *d;
>         unsigned long table_base;
>         unsigned long v;
> +       u32 high32;
>
> -       if (!(selector & ~3))
> +       if (!(selector & ~SEGMENT_RPL_MASK))
>                 return 0;
>
> -       table_base = get_current_gdt_rw_vaddr();
> -
> -       if (selector & 4) {           /* from ldt */
> +       /* LDT selector */
> +       if ((selector & SEGMENT_TI_MASK) == SEGMENT_LDT) {
>                 u16 ldt_selector = kvm_read_ldt();
>
> -               if (!(ldt_selector & ~3))
> +               if (!(ldt_selector & ~SEGMENT_RPL_MASK))
>                         return 0;
>
>                 table_base = segment_base(ldt_selector);
> +       } else {
> +               table_base = get_current_gdt_rw_vaddr();
>         }
> -       d = (struct desc_struct *)(table_base + (selector & ~7));
> +
> +       d = (struct desc_struct *)table_base + (selector >> 3);
>         v = get_desc_base(d);
>  #ifdef CONFIG_X86_64
> -       if (d->s == 0 && (d->type == 2 || d->type == 9 || d->type == 11))
> -               v |= ((unsigned long)((struct ldttss_desc64 *)d)->base3) << 
> 32;
> +       /*
> +        * Extend the virtual address if we have a system descriptor entry for
> +        * LDT or TSS (available or busy).
> +        */
> +       if (d->s == 0 && (d->type == DESC_LDT || d->type == DESC_TSS ||
> +                         d->type == 11/*Busy TSS */)) {
> +               high32 = ((struct ldttss_desc64 *)d)->base3;
> +               v |= (u64)high32 << 32;
> +       }
>  #endif
>         return v;
>  }
> --
> 2.11.0.483.g087da7b7c-goog
>

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