Re: [PATCH v3] x86/hyperv: use dynamically allocated page for hypercalls

[Alejandro Vallejo <agarciav@xxxxxxx>]

On Mon Apr 28, 2025 at 8:57 PM BST, 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.
>
> 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>

  Reviewed-by: Alejandro Vallejo <agarciav@xxxxxxx>

I'm happy with the patch as-is, and I'm equally happy...

> diff --git a/xen/arch/x86/guest/hyperv/hyperv.c 
> b/xen/arch/x86/guest/hyperv/hyperv.c
> index 6989af38f1..f69f596441 100644
> --- a/xen/arch/x86/guest/hyperv/hyperv.c
> +++ b/xen/arch/x86/guest/hyperv/hyperv.c
> @@ -98,10 +98,18 @@ 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\n");

... with Roger's suggestion to probe <4G first with MEMF(32) here.

> +
> +        mfn = virt_to_mfn(hcall_page);
>          hypercall_msr.enable = 1;
>          hypercall_msr.guest_physical_address = mfn;
>          wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> +
> +        dprintk(XENLOG_INFO,
> +                "Hyper-V: Allocated hypercall page at MFN %lx\n", mfn);

nit: This is a personal preference thing, but I find %lx is very
unhelpful when the variable is something like 0x123, as 123 might be
misinterpreted to be in decimal. An mfn is not nearly as problematic as
an error code, but %#lx avoids the ambiguity altogether.

Cheers,
Alejandro