RE: [PATCH] supporting non-NX/XD capable processors on x86_64 xenlinux

RE: [PATCH] supporting non-NX/XD capable processors on x86_64 xenlinux

[Nakajima, Jun]

Keir Fraser wrote:
> On 18 Aug 2005, at 05:29, Nakajima, Jun wrote:
> 
>> I don't have particular preference, but the latter one's change is
>> smaller, and it can fix other places (that are using the attrributes
>> derived from __PAGE_KERNEL). But the former one is cleaner to me. If
>> people agree, I can make such a patch.
> 
> How does native x86/64 Linux avoid this problem? It doesn't mask
> __PAGE_KERNEL with __supported_pte_mask, so I'm reluctant to take this
> patch. Seems to me there is some other underlying difference between
> us and native that is not best solved by diverging even further.
> 
> Unless native doesn;t boot on these failing boxes either?
> 
>   -- Keir

One obvious reason is: xen-specific drivers are using __PAGE_KERNEL (and
they don't exist in the native Linux). If xen_contig_memory, for
example, does not work, it won't boot... Fixing xen_contig_memory did
not fix the booting problem as long as I remember? Since then we fixed
some attribute problems in other xen-specific drivers when we brough up
domU, so I'll take a look.

xen_contig_memory
...
    /* 3. Map the new extent in place of old pages. */
    for (i = 0; i < (1<<order); i++) {
        BUG_ON(HYPERVISOR_update_va_mapping(
            vstart + (i*PAGE_SIZE),
            __pte_ma(((mfn+i)<<PAGE_SHIFT)|__PAGE_KERNEL), 0));
        xen_machphys_update(mfn+i, (__pa(vstart)>>PAGE_SHIFT)+i);
        phys_to_machine_mapping[(__pa(vstart)>>PAGE_SHIFT)+i] = mfn+i;
    }


blkback.c:  dispatch_probe
static void dispatch_probe(blkif_t *blkif, blkif_request_t *req)
{
 ...
#ifdef CONFIG_XEN_BLKDEV_TAP_BE
    if ( HYPERVISOR_update_va_mapping_otherdomain(
        MMAP_VADDR(pending_idx, 0),
        (pte_t) { (req->frame_and_sects[0] & PAGE_MASK) | __PAGE_KERNEL
},
        0, (blkif->is_blktap ? ID_TO_DOM(req->id) : blkif->domid) ) )

        goto out;
#else
    if ( HYPERVISOR_update_va_mapping_otherdomain(
        MMAP_VADDR(pending_idx, 0),
        (pte_t) { (req->frame_and_sects[0] & PAGE_MASK) | __PAGE_KERNEL
},
        0, blkif->domid) )

        goto out;
#endif
#endif /* endif CONFIG_XEN_BLKDEV_GRANT */

Jun
---
Intel Open Source Technology Center

Re: [PATCH] supporting non-NX/XD capable processors on x86_64 xenlinux

[Keir Fraser]

On 18 Aug 2005, at 15:15, Nakajima, Jun wrote:

>
> One obvious reason is: xen-specific drivers are using __PAGE_KERNEL 
> (and
> they don't exist in the native Linux). If xen_contig_memory, for
> example, does not work, it won't boot... Fixing xen_contig_memory did
> not fix the booting problem as long as I remember?

Yeah, I see what you mean. I've just applied a patch that cleans up the 
few remaining bad spots where we weren't using pfn_pte() or 
pfn_pte_ma() constructors. I can now boot okay even with NX/XD disabled 
in Xen.

By the way -- from what I've seen of this NX/XD discussion so far, it 
sounds like even if CPUID says that NX/XD is supported, and you set the 
EFER.NXE bit, the BIOS can still cause this to be ignored! Is this true 
and, if so, what will happen if you try to use NX/XD anyway? i.e., will 
the bit be ignored, or will its use cause a page fault?

  -- Keir

Re: [PATCH] supporting non-NX/XD capable processors on x86_64 xenlinux

[Jerone Young]

On Thu, 2005-08-18 at 16:09 +0100, Keir Fraser wrote:
> On 18 Aug 2005, at 15:15, Nakajima, Jun wrote:
> 
> >
> > One obvious reason is: xen-specific drivers are using __PAGE_KERNEL 
> > (and
> > they don't exist in the native Linux). If xen_contig_memory, for
> > example, does not work, it won't boot... Fixing xen_contig_memory did
> > not fix the booting problem as long as I remember?
> 
> Yeah, I see what you mean. I've just applied a patch that cleans up the 
> few remaining bad spots where we weren't using pfn_pte() or 
> pfn_pte_ma() constructors. I can now boot okay even with NX/XD disabled 
> in Xen.
> 
> By the way -- from what I've seen of this NX/XD discussion so far, it 
> sounds like even if CPUID says that NX/XD is supported, and you set the 
> EFER.NXE bit, the BIOS can still cause this to be ignored!

If I remember correctly (will create a patch shortly to make 100% sure).
If the option is off in the BIOS than NX bit will not show up as a CPU
feature flag. And of course once it's on in the bios then it does.

>  Is this true 
> and, if so, what will happen if you try to use NX/XD anyway? i.e., will 
> the bit be ignored, or will its use cause a page fault?
> 
>   -- Keir
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
> 
-- 
Jerone Young
IBM Linux Technology Center
jyoung5@us.ibm.com
512-838-1157 (T/L: 678-1157)

Re: [PATCH] supporting non-NX/XD capable processors on x86_64 xenlinux

[Andi Kleen]

Keir Fraser <Keir.Fraser@cl.cam.ac.uk> writes:
> 
> By the way -- from what I've seen of this NX/XD discussion so far, it
> sounds like even if CPUID says that NX/XD is supported, and you set
> the EFER.NXE bit, the BIOS can still cause this to be ignored! Is this
> true and, if so, what will happen if you try to use NX/XD anyway?
> i.e., will the bit be ignored, or will its use cause a page fault?

I don't think that can happen. When NX is disabled or not there it
doesn't appear in CPUID.

-Andi

Re: Re: [PATCH] supporting non-NX/XD capable processors on x86_64 xenlinux

[Jerone Young]

On Thu, 2005-08-18 at 17:51 +0200, Andi Kleen wrote:
> Keir Fraser <Keir.Fraser@cl.cam.ac.uk> writes:
> > 
> > By the way -- from what I've seen of this NX/XD discussion so far, it
> > sounds like even if CPUID says that NX/XD is supported, and you set
> > the EFER.NXE bit, the BIOS can still cause this to be ignored! Is this
> > true and, if so, what will happen if you try to use NX/XD anyway?
> > i.e., will the bit be ignored, or will its use cause a page fault?
> 
> I don't think that can happen. When NX is disabled or not there it
> doesn't appear in CPUID.

Ah but it does:

With NX bit cut "on" in bios:
/proc/cpuinfo
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 15
model           : 4
model name      :                   Intel(R) Xeon(TM) CPU 3.20GHz
stepping        : 1
cpu MHz         : 800.028
cache size      : 1024 KB
physical id     : 0
siblings        : 2
core id         : 0
cpu cores       : 1
fpu             : yes
fpu_exception   : yes
cpuid level     : 5
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm syscall
nx lm constant_tsc pni monitor ds_cpl cid cx16 xtpr   <--- NX found here
bogomips        : 6396.31
clflush size    : 64
cache_alignment : 128
address sizes   : 36 bits physical, 48 bits virtual
power management:

With NX bit cut "off" in bios:
cat /proc/cpuinfo
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 15
model           : 4
model name      :                   Intel(R) Xeon(TM) CPU 3.20GHz
stepping        : 1
cpu MHz         : 3200.280
cache size      : 1024 KB
physical id     : 0
siblings        : 2
core id         : 0
cpu cores       : 1
fpu             : yes
fpu_exception   : yes
cpuid level     : 5
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm syscall
lm pni monitor ds_cpl cid cmpxchg16b
bogomips        : 6324.22
clflush size    : 64
cache_alignment : 128
address sizes   : 36 bits physical, 48 bits virtual
power management:

Notice no nx in the flags when NX is turned off in the bios. 




> 
> -Andi
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
> 
-- 
Jerone Young
IBM Linux Technology Center
jyoung5@us.ibm.com
512-838-1157 (T/L: 678-1157)

Re: [PATCH] supporting non-NX/XD capable processors on x86_64 xenlinux

[Keir Fraser]

On 18 Aug 2005, at 16:39, Jerone Young wrote:

> If I remember correctly (will create a patch shortly to make 100% 
> sure).
> If the option is off in the BIOS than NX bit will not show up as a CPU
> feature flag. And of course once it's on in the bios then it does.

Good to hear. Case closed on nx/xd then...

  -- Keir

Re: Re: [PATCH] supporting non-NX/XD capable processors on x86_64 xenlinux

[Andi Kleen]

On Thu, Aug 18, 2005 at 11:05:42AM -0500, Jerone Young wrote:
> On Thu, 2005-08-18 at 17:51 +0200, Andi Kleen wrote:
> > Keir Fraser <Keir.Fraser@cl.cam.ac.uk> writes:
> > > 
> > > By the way -- from what I've seen of this NX/XD discussion so far, it
> > > sounds like even if CPUID says that NX/XD is supported, and you set
> > > the EFER.NXE bit, the BIOS can still cause this to be ignored! Is this
> > > true and, if so, what will happen if you try to use NX/XD anyway?
> > > i.e., will the bit be ignored, or will its use cause a page fault?
> > 
> > I don't think that can happen. When NX is disabled or not there it
> > doesn't appear in CPUID.
> 
> Ah but it does:

Your example confirms what I said.

-Andi

Re: Re: [PATCH] supporting non-NX/XD capable processors on x86_64 xenlinux

[Jerone Young]

On Thu, 2005-08-18 at 18:18 +0200, Andi Kleen wrote:
> On Thu, Aug 18, 2005 at 11:05:42AM -0500, Jerone Young wrote:
> > On Thu, 2005-08-18 at 17:51 +0200, Andi Kleen wrote:
> > > Keir Fraser <Keir.Fraser@cl.cam.ac.uk> writes:
> > > > 
> > > > By the way -- from what I've seen of this NX/XD discussion so far, it
> > > > sounds like even if CPUID says that NX/XD is supported, and you set
> > > > the EFER.NXE bit, the BIOS can still cause this to be ignored! Is this
> > > > true and, if so, what will happen if you try to use NX/XD anyway?
> > > > i.e., will the bit be ignored, or will its use cause a page fault?
> > > 
> > > I don't think that can happen. When NX is disabled or not there it
> > > doesn't appear in CPUID.
> > 
> > Ah but it does:

hehe... I didn't realize I left that statement in there...I just wanted
to post to the list to confirm with an example. Don't worry Andi ...
your still the x86-64 god in my book!

> 
> Your example confirms what I said.

> 
> -Andi
> 
-- 
Jerone Young
IBM Linux Technology Center
jyoung5@us.ibm.com
512-838-1157 (T/L: 678-1157)