Re: [PATCH 2/2] hyperv: Do not overlap the input and output hypercall areas in get_vtl(void)

[Wei Liu <wei.liu@kernel.org>]

On Wed, Dec 18, 2024 at 12:54:21PM -0800, Roman Kisel wrote:
> The Top-Level Functional Specification for Hyper-V, Section 3.6 [1, 2], disallows
> overlapping of the input and output hypercall areas, and get_vtl(void) does
> overlap them.
> 
> To fix this, enable allocation of the output hypercall pages when running in
> the VTL mode and use the output hypercall page of the current vCPU for the
> hypercall.
> 
> [1] https://learn.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/hypercall-interface
> [2] https://github.com/MicrosoftDocs/Virtualization-Documentation/tree/main/tlfs
> 
> Fixes: 8387ce06d70b ("x86/hyperv: Set Virtual Trust Level in VMBus init message")
> Signed-off-by: Roman Kisel <romank@linux.microsoft.com>
> ---
>  arch/x86/hyperv/hv_init.c | 2 +-
>  drivers/hv/hv_common.c    | 6 +++---
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index c7185c6a290b..90c9ea00273e 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -422,7 +422,7 @@ static u8 __init get_vtl(void)
>  
>  	local_irq_save(flags);
>  	input = *this_cpu_ptr(hyperv_pcpu_input_arg);
> -	output = (struct hv_get_vp_registers_output *)input;
> +	output = *this_cpu_ptr(hyperv_pcpu_output_arg);

You can do

	output = (char *)input + HV_HYP_PAGE_SIZE / 2;

to avoid the extra allocation.

The input and output structures surely won't take up half of the page.

Thanks,
Wei.

>  
>  	memset(input, 0, struct_size(input, names, 1));
>  	input->partition_id = HV_PARTITION_ID_SELF;
> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
> index c4fd07d9bf1a..5178beed6ca8 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)) {
>  		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
>