Re: [Xen-devel] [PATCH] x86: correct CPUID output for out of bounds input

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 24/08/16 16:31, Jan Beulich wrote:
> Another place where we should try to behave like real hardware; see
> the code comments.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -3358,6 +3358,31 @@ void hvm_cpuid(unsigned int input, unsig
>      if ( !edx )
>          edx = &dummy;
>  
> +    if ( input & 0xffff )
> +    {
> +        /*
> +         * Requests beyond the highest supported leaf within a group return
> +         * zero on AMD and the highest basic leaf output on others.
> +         */
> +        unsigned int lvl;
> +
> +        hvm_cpuid(input & 0xffff0000, &lvl, NULL, NULL, NULL);

I have specifically deferred fixing this issue so far, because I don't
want to increase the quantity of recursion with hvm_cpuid().

Also, because of the poor datastructure for domain cpuid, this adds 1
and possibly 2 extra loops over the unordered list.


On the way back from Toronto, I started experimenting with my
full-policy plans, including a structured information layout so
cpuid.basic.max_leaf can be found directly, and starting a guest_cpuid()
function intending to replace both pv_cpuid() and hvm_cpuid() in due course.

Would you be amenable to leaving this issue as-is for now, until there
is a more efficient way of fixing it?

~Andrew

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