Re: [PATCH v5 3/5] hyperv: Enable the hypercall output page for the VTL mode

[Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>]

On Mon, Dec 30, 2024 at 10:09:39AM -0800, Roman Kisel wrote:
> Due to the hypercall page not being allocated in the VTL mode,
> the code resorts to using a part of the input page.
> 
> Allocate the hypercall output page in the VTL mode thus enabling
> it to use it for output and share code with dom0.
> 
> Signed-off-by: Roman Kisel <romank@linux.microsoft.com>
> ---
>  drivers/hv/hv_common.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
> index c6ed3ba4bf61..c983cfd4d6c0 100644
> --- a/drivers/hv/hv_common.c
> +++ b/drivers/hv/hv_common.c
> @@ -340,7 +340,7 @@ int __init hv_common_init(void)
>  	BUG_ON(!hyperv_pcpu_input_arg);
>  
>  	/* Allocate the per-CPU state for output arg for root */
> -	if (hv_root_partition) {
> +	if (hv_root_partition || IS_ENABLED(CONFIG_HYPERV_VTL_MODE)) {

This check doesn't look nice.
First of all, IS_ENABLED(CONFIG_HYPERV_VTL_MODE) doesn't mean that this
particular kernel is being booted in VTL other that VTL0.

Second, current approach is that a VTL1+ kernel is a different build from VTL0
kernel and thus relying on the config option looks reasonable. However,
this is not true in general case.

I'd suggest one of the following three options:

1. Always allocate per-cpu output pages. This is wasteful for the VTL0
guest case, but it might worth it for overall simplification.

2. Don't touch this code and keep the cnage in the Underhill tree.

3. Introduce a configuration option (command line or device tree or
both) and use it instead of the kernel config option.

Thanks,
Stas

>  		hyperv_pcpu_output_arg = alloc_percpu(void *);
>  		BUG_ON(!hyperv_pcpu_output_arg);
>  	}
> @@ -435,7 +435,7 @@ int hv_common_cpu_init(unsigned int cpu)
>  	void **inputarg, **outputarg;
>  	u64 msr_vp_index;
>  	gfp_t flags;
> -	int pgcount = hv_root_partition ? 2 : 1;
> +	const int pgcount = (hv_root_partition || IS_ENABLED(CONFIG_HYPERV_VTL_MODE)) ? 2 : 1;
>  	void *mem;
>  	int ret;
>  
> @@ -453,7 +453,7 @@ int hv_common_cpu_init(unsigned int cpu)
>  		if (!mem)
>  			return -ENOMEM;
>  
> -		if (hv_root_partition) {
> +		if (hv_root_partition || IS_ENABLED(CONFIG_HYPERV_VTL_MODE)) {
>  			outputarg = (void **)this_cpu_ptr(hyperv_pcpu_output_arg);
>  			*outputarg = (char *)mem + HV_HYP_PAGE_SIZE;
>  		}
> -- 
> 2.34.1
>