RE: [PATCH 1/2] hyperv: Move hv_current_partition_id to arch-generic code

[Michael Kelley]

From: Nuno Das Neves <nunodasneves@xxxxxxxxxxxxxxxxxxx> Sent: Friday, December 6, 2024 2:22 PM
> 
> Make hv_current_partition_id available in both x86_64 and arm64.
> This feature isn't specific to x86_64 and will be needed by common
> code.
> 
> While at it, replace the BUG()s with WARN()s. Failing to get the id
> need not crash the machine (although it is a very bad sign).
> 
> Signed-off-by: Nuno Das Neves <nudasnev@xxxxxxxxxxxxx>
> ---
>  arch/arm64/hyperv/mshyperv.c    |  3 +++
>  arch/x86/hyperv/hv_init.c       | 25 +------------------------
>  arch/x86/include/asm/mshyperv.h |  2 --
>  drivers/hv/hv_common.c          | 23 +++++++++++++++++++++++
>  include/asm-generic/mshyperv.h  |  2 ++
>  5 files changed, 29 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c
> index b1a4de4eee29..5050e748d266 100644
> --- a/arch/arm64/hyperv/mshyperv.c
> +++ b/arch/arm64/hyperv/mshyperv.c
> @@ -19,6 +19,9 @@
> 
>  static bool hyperv_initialized;
> 
> +u64 hv_current_partition_id = HV_PARTITION_ID_SELF;
> +EXPORT_SYMBOL_GPL(hv_current_partition_id);
> +

Instead of adding a definition of hv_current_partition_id on
the arm64 side, couldn't the definition on the x86 side in
hv_init.c be moved to hv_common.c (or maybe somewhere
else that is specific to running in the root partition, per my
comments in the cover letter), so there is only one definition
shared by both architectures?

>  int hv_get_hypervisor_version(union hv_hypervisor_version_info *info)
>  {
>  	hv_get_vpreg_128(HV_REGISTER_HYPERVISOR_VERSION,
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index 95eada2994e1..950f5ccdb9d9 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -35,7 +35,7 @@
>  #include <clocksource/hyperv_timer.h>
>  #include <linux/highmem.h>
> 
> -u64 hv_current_partition_id = ~0ull;
> +u64 hv_current_partition_id = HV_PARTITION_ID_SELF;
>  EXPORT_SYMBOL_GPL(hv_current_partition_id);
> 
>  void *hv_hypercall_pg;
> @@ -394,24 +394,6 @@ static void __init hv_stimer_setup_percpu_clockev(void)
>  		old_setup_percpu_clockev();
>  }
> 
> -static void __init hv_get_partition_id(void)
> -{
> -	struct hv_get_partition_id *output_page;
> -	u64 status;
> -	unsigned long flags;
> -
> -	local_irq_save(flags);
> -	output_page = *this_cpu_ptr(hyperv_pcpu_output_arg);
> -	status = hv_do_hypercall(HVCALL_GET_PARTITION_ID, NULL, output_page);
> -	if (!hv_result_success(status)) {
> -		/* No point in proceeding if this failed */
> -		pr_err("Failed to get partition ID: %lld\n", status);
> -		BUG();
> -	}
> -	hv_current_partition_id = output_page->partition_id;
> -	local_irq_restore(flags);
> -}
> -
>  #if IS_ENABLED(CONFIG_HYPERV_VTL_MODE)
>  static u8 __init get_vtl(void)
>  {
> @@ -606,11 +588,6 @@ void __init hyperv_init(void)
> 
>  	register_syscore_ops(&hv_syscore_ops);
> 
> -	if (cpuid_ebx(HYPERV_CPUID_FEATURES) & HV_ACCESS_PARTITION_ID)
> -		hv_get_partition_id();
> -
> -	BUG_ON(hv_root_partition && hv_current_partition_id == ~0ull);
> -
>  #ifdef CONFIG_PCI_MSI
>  	/*
>  	 * If we're running as root, we want to create our own PCI MSI domain.
> diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
> index 5f0bc6a6d025..9eeca2a6d047 100644
> --- a/arch/x86/include/asm/mshyperv.h
> +++ b/arch/x86/include/asm/mshyperv.h
> @@ -44,8 +44,6 @@ extern bool hyperv_paravisor_present;
> 
>  extern void *hv_hypercall_pg;
> 
> -extern u64 hv_current_partition_id;
> -
>  extern union hv_ghcb * __percpu *hv_ghcb_pg;
> 
>  bool hv_isolation_type_snp(void);
> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
> index 7a35c82976e0..819bcfd2b149 100644
> --- a/drivers/hv/hv_common.c
> +++ b/drivers/hv/hv_common.c
> @@ -278,11 +278,34 @@ static void hv_kmsg_dump_register(void)
>  	}
>  }
> 
> +static void __init hv_get_partition_id(void)
> +{
> +	struct hv_get_partition_id *output_page;
> +	u64 status;
> +	unsigned long flags;
> +
> +	local_irq_save(flags);
> +	output_page = *this_cpu_ptr(hyperv_pcpu_output_arg);
> +	status = hv_do_hypercall(HVCALL_GET_PARTITION_ID, NULL, output_page);
> +	if (!hv_result_success(status)) {
> +		local_irq_restore(flags);
> +		WARN(true, "Failed to get partition ID: %lld\n", status);
> +		return;
> +	}
> +	hv_current_partition_id = output_page->partition_id;
> +	local_irq_restore(flags);
> +}
> +
>  int __init hv_common_init(void)
>  {
>  	int i;
>  	union hv_hypervisor_version_info version;
> 
> +	if (ms_hyperv.priv_high & HV_ACCESS_PARTITION_ID)
> +		hv_get_partition_id();

hv_get_partition_id() uses the hyperv_pcpu_output_arg, and at
this point, hyperv_pcpu_output_arg isn't set. That setup
is done later in hv_common_init().

> +
> +	WARN_ON(hv_root_partition && hv_current_partition_id == HV_PARTITION_ID_SELF);
> +

Since the hypercall will fail cleanly if the calling VM doesn't
have the HV_ACCESS_PARTITION_ID privilege, could the
above be simplified to just this?

	if (hv_root_partition)
		hv_get_partition_id():

A non-root partition VM doesn't need to get the partition ID, while a
root partition should have the privilege. If the hypercall fails, there's
already a WARN, so there's no value in doing another WARN. Also if
the hypercall succeeds, it presumably returns a specific partitionID, not
HV_PARTITION_ID_SELF, so we know we have what we want.

There's already an "if (hv_root_partition)" statement for setting up
the hyperv_pcpu_output_arg. The call to hv_get_partition_id() could
go under that existing "if" *after* the hyperv_pcpu_output_arg is
set. :-)

Michael

>  	/* Get information about the Hyper-V host version */
>  	if (!hv_get_hypervisor_version(&version))
>  		pr_info("Hyper-V: Host Build %d.%d.%d.%d-%d-%d\n",
> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
> index 8fe7aaab2599..8c4ff6e9aae7 100644
> --- a/include/asm-generic/mshyperv.h
> +++ b/include/asm-generic/mshyperv.h
> @@ -60,6 +60,8 @@ struct ms_hyperv_info {
>  extern struct ms_hyperv_info ms_hyperv;
>  extern bool hv_nested;
> 
> +extern u64 hv_current_partition_id;
> +
>  extern void * __percpu *hyperv_pcpu_input_arg;
>  extern void * __percpu *hyperv_pcpu_output_arg;
> 
> --
> 2.34.1