Re: dom0 PV looping on search_pre_exception_table()

[Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>]

On Thu, Dec 10, 2020 at 09:01:12PM +0000, Andrew Cooper wrote:
> I've repro'd the problem.
> 
> When I modify Xen to explicitly demand-map the LDT in the MMUEXT_SET_LDT
> hypercall, everything works fine.
> 
> Specifically, this delta:
> 
> diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
> index 723cc1070f..71a791d877 100644
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -3742,12 +3742,31 @@ long do_mmuext_op(
>              else if ( (curr->arch.pv.ldt_ents != ents) ||
>                        (curr->arch.pv.ldt_base != ptr) )
>              {
> +                unsigned int err = 0, tmp;
> +
>                  if ( pv_destroy_ldt(curr) )
>                      flush_tlb_local();
>  
>                  curr->arch.pv.ldt_base = ptr;
>                  curr->arch.pv.ldt_ents = ents;
>                  load_LDT(curr);
> +
> +                printk("Probe new LDT\n");
> +                asm volatile (
> +                    "mov %%es, %[tmp];\n\t"
> +                    "1: mov %[sel], %%es;\n\t"
> +                    "mov %[tmp], %%es;\n\t"
> +                    "2:\n\t"
> +                    ".section .fixup,\"ax\"\n"
> +                    "3: mov $1, %[err];\n\t"
> +                    "jmp 2b\n\t"
> +                    ".previous\n\t"
> +                    _ASM_EXTABLE(1b, 3b)
> +                    : [err] "+r" (err),
> +                      [tmp] "=&r" (tmp)
> +                    : [sel] "r" (0x3f)
> +                    : "memory");
> +                printk("  => err %u\n", err);
>              }
>              break;
>          }
> 
> Which stashes %es, explicitly loads init's %ss selector to trigger the
> #PF and Xen's lazy mapping, then restores %es.

Yes, this works for dom0 too, I have it running multiuser

> [...]
> 
> Presumably you've got no Meltdown mitigations going on within the NetBSD
> kernel?  (I suspect not, seeing as changing Xen changes the behaviour,
> but it is worth asking).

No, there's no Meltdown mitigations for PV in NetBSD. as I see it,
for amd64 at last, the Xen kernel has to do it anyway, so it's not usefull
to implement it in the guest's kernel. Did I miss something ?

-- 
Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>
     NetBSD: 26 ans d'experience feront toujours la difference
--