Re: Xen-Error: Disabling IOMMU on Stepping C2 5520 Host-Bridge

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Sun, Jul 25, 2021 at 02:31:17PM +0200, luja wrote:
> Hi Marek, Hi all,

Hi luja,

First of all, please use appropriate mailing list for such emails, not
email individual developers privately. I'm adding xen-devel here.

> 
> On a HP Z600 I am trying to run qubes.
> The Xen log says that the Chipset is affected by Intel-Errate #47, #53
> 
> the code in Xen is this:
> 
> "
> /* 5500/5520/X58 Chipset Interrupt remapping errata, for stepping B-3.
> * Fixed in stepping C-2. */
> static void __init tylersburg_intremap_quirk(void)
> {
> uint32_t bus, device;
> uint8_t rev;
> 
> for ( bus = 0; bus < 0x100; bus++ )
> {
> /* Match on System Management Registers on Device 20 Function 0 */
> device = pci_conf_read32(0, bus, 20, 0, PCI_VENDOR_ID);
> rev = pci_conf_read8(0, bus, 20, 0, PCI_REVISION_ID);
> 
> if ( rev == 0x13 && device == 0x342e8086 )
> {
> printk(XENLOG_WARNING VTDPREFIX
> "Disabling IOMMU due to Intel 5500/5520/X58 Chipset errata #47, #53\n");
> iommu_enable = 0;
> break;
> }
> }
> }
> 
> "
> 
> But! rev 0x13 is not suficient to detect the "wrong" host bridge.

According to the spec by Intel (page 11 in the PDF you attached), it is.

> This Z600 is equipped with 0B54h mainboard as can be seen with dmi-decode.
> 
> The manual states that 0B54h mainboard has the "newer C2 stepping",
> so it is *not* affected by Intel "spec update" (nota bene: Intel updates the
> spec, others report erratas) bugs  

The code above checks for rev 0x13, and the spec (page 11) clearly says that rev
0x13 is stepping B-3. Stepping C-2 is rev 0x22. So, if this check
triggers for you, I'm afraid you have the affected chipset.

According to HP doc you attached, you can additionally confirm it via
BIOS:
    To determine if a specific HP Z600 system
    has the C2 revision of the chipset:
    1. Use the BIOS setup menu to access the “Boot
    Block Date” from the “System Information Menu.”
    All B3-based systems will have a “1/30/09”
    date and C2-based systems will have a
    “01/07/10” date.

> So the way Xen detects the "bug" (pci rev 13) is not sufficient, as my Z600
> shows pci rev13 with lspci but 0xB54h (board rev only on Z600) with dmidecode
> I would suggest first to have an override xen kernel boot option to disable 
> the disablement in this code section. Or just patch this part out of the Xen 
> code and rebuild xen. If this stuff really crashes, one will see it.

Patching it out is out of the question, this check if there for a
reason.

> So please build a new xen without this stupid disablement or please add an 
> override boot command for it.
> 
> Please see the attached upgrade manual of Z600 and the errata "spec update" 
> by Intel.
> You see that the C2 stepping is not affected by the bugs refered to in the 
> xen code,
> so removing that section or adding better detection of the mask revision (B3 
> vs. C2)  of 5520 host bridge would allow  many users to operate Qubes4.

Maybe someone else has an alternative idea?

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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