Re: [Xen-devel] [PATCH v9 07/11] x86/boot: Calculate the most appropriate BTI mitigation to use

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.01.18 at 16:46, <andrew.cooper3@xxxxxxxxxx> wrote:
> +/* Calculate whether Retpoline is known-safe on this CPU. */
> +static bool __init retpoline_safe(void)
> +{
> +    unsigned int ucode_rev = this_cpu(ucode_cpu_info).cpu_sig.rev;
> +
> +    if ( boot_cpu_data.x86_vendor == X86_VENDOR_AMD )
> +        return true;
> +
> +    if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
> +         boot_cpu_data.x86 != 6 )
> +        return false;
> +
> +    switch ( boot_cpu_data.x86_model )
> +    {
> +    case 0x17: /* Penryn */
> +    case 0x1d: /* Dunnington */
> +    case 0x1e: /* Nehalem */
> +    case 0x1f: /* Auburndale / Havendale */
> +    case 0x1a: /* Nehalem EP */
> +    case 0x2e: /* Nehalem EX */
> +    case 0x25: /* Westmere */
> +    case 0x2c: /* Westmere EP */
> +    case 0x2f: /* Westmere EX */
> +    case 0x2a: /* SandyBridge */
> +    case 0x2d: /* SandyBridge EP/EX */
> +    case 0x3a: /* IvyBridge */
> +    case 0x3e: /* IvyBridge EP/EX */
> +    case 0x3c: /* Haswell */
> +    case 0x3f: /* Haswell EX/EP */
> +    case 0x45: /* Haswell D */
> +    case 0x46: /* Haswell H */
> +        return true;

Especially regarding this part of the function (leave the other half
in context for reference) - can we perhaps shorten this? E.g.
fold together everything below the lowest Broadwell model value?
There in particular don't appear to be an Atoms in the list above,
yet either they're affected by the underlying issue as well (and
hence would benefit from whatever applicable mitigation), or we
should suppress as much mitigation overhead as possible for them
(which I don't think is the case).

In any event, as said before - this function will need an ack from
an Intel person, and it would be nice for ...

> +        /*
> +         * Broadwell processors are retpoline-safe after specific microcode
> +         * versions.
> +         */
> +    case 0x3d: /* Broadwell */
> +        return ucode_rev >= 0x28;
> +    case 0x47: /* Broadwell H */
> +        return ucode_rev >= 0x1b;
> +    case 0x4f: /* Broadwell EP/EX */
> +        return ucode_rev >= 0xb000025;
> +    case 0x56: /* Broadwell D */
> +        return false; /* TBD. */

... this to be filled in.

Jan

> +        /*
> +         * Skylake and later processors are not retpoline-safe.
> +         */
> +    default:
> +        return false;
> +    }
>  }



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