RE: [PATCH v5 06/16] x86/hyperv: allocate output arg pages if required

Michael Kelley
2021-01-26T10:34:12Z

From: Wei Liu <wei.liu@kernel.org> Sent: Wednesday, January 20, 2021 4:01 AM
>
> When Linux runs as the root partition, it will need to make hypercalls
> which return data from the hypervisor.
>
> Allocate pages for storing results when Linux runs as the root
> partition.
>
> Signed-off-by: Lillian Grassin-Drake <ligrassi@microsoft.com>
> Co-Developed-by: Lillian Grassin-Drake <ligrassi@microsoft.com>
> Signed-off-by: Wei Liu <wei.liu@kernel.org>
> ---
> v3: Fix hv_cpu_die to use free_pages.
> v2: Address Vitaly's comments
> ---
> arch/x86/hyperv/hv_init.c | 35 ++++++++++++++++++++++++++++-----
> arch/x86/include/asm/mshyperv.h | 1 +
> 2 files changed, 31 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index e04d90af4c27..6f4cb40e53fe 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -41,6 +41,9 @@ EXPORT_SYMBOL_GPL(hv_vp_assist_page);
> void __percpu **hyperv_pcpu_input_arg;
> EXPORT_SYMBOL_GPL(hyperv_pcpu_input_arg);
>
> +void __percpu **hyperv_pcpu_output_arg;
> +EXPORT_SYMBOL_GPL(hyperv_pcpu_output_arg);
> +
> u32 hv_max_vp_index;
> EXPORT_SYMBOL_GPL(hv_max_vp_index);
>
> @@ -73,12 +76,19 @@ static int hv_cpu_init(unsigned int cpu)
> void **input_arg;
> struct page *pg;
>
> - input_arg = (void **)this_cpu_ptr(hyperv_pcpu_input_arg);
> /* hv_cpu_init() can be called with IRQs disabled from hv_resume() */
> - pg = alloc_page(irqs_disabled() ? GFP_ATOMIC : GFP_KERNEL);
> + pg = alloc_pages(irqs_disabled() ? GFP_ATOMIC : GFP_KERNEL, hv_root_partition ?
> 1 : 0);
> if (unlikely(!pg))
> return -ENOMEM;
> +
> + input_arg = (void **)this_cpu_ptr(hyperv_pcpu_input_arg);
> *input_arg = page_address(pg);
> + if (hv_root_partition) {
> + void **output_arg;
> +
> + output_arg = (void **)this_cpu_ptr(hyperv_pcpu_output_arg);
> + *output_arg = page_address(pg + 1);
> + }
>
> hv_get_vp_index(msr_vp_index);
>
> @@ -205,14 +215,23 @@ static int hv_cpu_die(unsigned int cpu)
> unsigned int new_cpu;
> unsigned long flags;
> void **input_arg;
> - void *input_pg = NULL;
> + void *pg;
>
> local_irq_save(flags);
> input_arg = (void **)this_cpu_ptr(hyperv_pcpu_input_arg);
> - input_pg = *input_arg;
> + pg = *input_arg;
> *input_arg = NULL;
> +
> + if (hv_root_partition) {
> + void **output_arg;
> +
> + output_arg = (void **)this_cpu_ptr(hyperv_pcpu_output_arg);
> + *output_arg = NULL;
> + }
> +
> local_irq_restore(flags);
> - free_page((unsigned long)input_pg);
> +
> + free_pages((unsigned long)pg, hv_root_partition ? 1 : 0);
>
> if (hv_vp_assist_page && hv_vp_assist_page[cpu])
> wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, 0);
> @@ -346,6 +365,12 @@ void __init hyperv_init(void)
>
> BUG_ON(hyperv_pcpu_input_arg == NULL);
>
> + /* Allocate the per-CPU state for output arg for root */
> + if (hv_root_partition) {
> + hyperv_pcpu_output_arg = alloc_percpu(void *);
> + BUG_ON(hyperv_pcpu_output_arg == NULL);
> + }
> +
> /* Allocate percpu VP index */
> hv_vp_index = kmalloc_array(num_possible_cpus(), sizeof(*hv_vp_index),
> GFP_KERNEL);
> diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
> index ac2b0d110f03..62d9390f1ddf 100644
> --- a/arch/x86/include/asm/mshyperv.h
> +++ b/arch/x86/include/asm/mshyperv.h
> @@ -76,6 +76,7 @@ static inline void hv_disable_stimer0_percpu_irq(int irq) {}
> #if IS_ENABLED(CONFIG_HYPERV)
> extern void *hv_hypercall_pg;
> extern void __percpu **hyperv_pcpu_input_arg;
> +extern void __percpu **hyperv_pcpu_output_arg;
>
> static inline u64 hv_do_hypercall(u64 control, void *input, void *output)
> {
> --
> 2.20.1

I think this all works OK. But a meta question: Do we need a separate
per-cpu output argument page? From the Hyper-V hypercall standpoint, I
don't think input and output args need to be in separate pages. They both
just need to not cross a page boundary. As long as we don't have a hypercall
where the sum of the sizes of the input and output args exceeds a page,
we could just have a single page, and split it up in any manner that works
for the particular hypercall.

Thoughts?

Michael