Re: [PATCH v2] x86/hyperv: use dynamically allocated page for hypercalls

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Apr 25, 2025 at 04:43:31PM -0700, Ariadne Conill wrote:
> Previously Xen placed the hypercall page at the highest possible MFN,
> but this caused problems on systems where there is more than 36 bits
> of physical address space.
> 
> In general, it also seems unreliable to assume that the highest possible
> MFN is not already reserved for some other purpose.
> 
> Changes from v1:
> - Continue to use fixmap infrastructure
> - Use panic in Hyper-V setup() function instead of returning -ENOMEM
>   on hypercall page allocation failure
> 
> Fixes: 620fc734f854 ("x86/hyperv: setup hypercall page")
> Cc: Alejandro Vallejo <agarciav@xxxxxxx>
> Cc: Alexander M. Merritt <alexander@xxxxxxxxx>
> Signed-off-by: Ariadne Conill <ariadne@ariadne.space>
> ---
>  xen/arch/x86/guest/hyperv/hyperv.c      | 17 +++++++----------
>  xen/arch/x86/include/asm/guest/hyperv.h |  3 ---
>  2 files changed, 7 insertions(+), 13 deletions(-)
> 
> diff --git a/xen/arch/x86/guest/hyperv/hyperv.c 
> b/xen/arch/x86/guest/hyperv/hyperv.c
> index 6989af38f1..0305374a06 100644
> --- a/xen/arch/x86/guest/hyperv/hyperv.c
> +++ b/xen/arch/x86/guest/hyperv/hyperv.c
> @@ -98,7 +98,13 @@ static void __init setup_hypercall_page(void)
>      rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
>      if ( !hypercall_msr.enable )
>      {
> -        mfn = HV_HCALL_MFN;
> +        void *hcall_page = alloc_xenheap_page();
> +        if ( !hcall_page )
> +            panic("Hyper-V: Failed to allocate hypercall trampoline page");
> +
> +        printk("Hyper-V: Allocated hypercall page @ %p.\n", hcall_page);

This likely wants to be a dprintk, and possibly also print the
physical address of the used page?  And no period at the end of the
sentence IMO.

I think Xen might have used the last page in the physical address
range to prevent HyperV from possibly shattering a superpage in the
second stage translation page-tables if normal RAM was used?

However I don't know whether HyperV will shatter super-pages if a
sub-page of it is used to contain the hypercall page (I don't think it
should?)

Thanks, Roger.