Re: [Xen-devel] [PATCH v8 15/15] xen: add new Xen cpuid node for max address width info

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.09.17 at 08:34, <jgross@xxxxxxxx> wrote:
> On very large hosts a guest needs to know whether it will have to

... a PV guest ...

> handle frame numbers larger than 32 bits in order to select the
> appropriate grant interface version.
> 
> Add a new Xen specific CPUID node to contain the maximum guest address
> width

"guest address width" is ambiguous here, the more when looking at
what you actually return. We should no longer allow ourselves to
mix up the different address spaces. The limit you want to report
here is that in MFN space, which ought to be of no relevance to
HVM guests. Therefore I'm against uniformly exposing this (as much
as almost no other host property should have any relevance for
HVM guests), and would instead like to see a PV-only leaf just like
we already have a HVM only one.

> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -929,6 +929,10 @@ void cpuid_hypervisor_leaves(const struct vcpu *v, 
> uint32_t leaf,
>          res->b = v->vcpu_id;
>          break;
>  
> +    case 5: /* Host specific parameters */
> +        res->a = generic_flsl(get_upper_mfn_bound() - 1) + PAGE_SHIFT;
> +        break;

Already when looking at the patch introducing the function I was
wondering whether the function wouldn't better return the highest
frame number instead of the first invalid one. From an abstract
perspective this would allow an arch (e.g. ARM32) to report that
all addresses are valid.

> --- a/xen/include/public/arch-x86/cpuid.h
> +++ b/xen/include/public/arch-x86/cpuid.h
> @@ -85,6 +85,15 @@
>  #define XEN_HVM_CPUID_IOMMU_MAPPINGS   (1u << 2)
>  #define XEN_HVM_CPUID_VCPU_ID_PRESENT  (1u << 3) /* vcpu id is present in 
> EBX 
> */
>  
> -#define XEN_CPUID_MAX_NUM_LEAVES 4
> +/*
> + * Leaf 6 (0x40000x05)
> + * Host specific parameters
> + * EAX: bits 0-7: max guest address width
> + */
> +
> +/* Max. address width in bits taking memory hotplug into account. */
> +#define XEN_CPUID_GUEST_ADDRESS_WIDTH_MASK (255u << 0)

Please use hex numbers for multi-digit masks, and please make
explicit (at least in the comment, perhaps also in the constant's
name) which "address" is being talked about here.

Jan


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