Linux Virtualization - Sep 2020 - [PATCH RFC v1 08/18] x86/hyperv: handling hypercall page setup for root

Wei Liu <wei.liu at kernel.org> writes:
> When Linux is running as the root partition, the hypercall page will
> have already been setup by Hyper-V. Copy the content over to the
> allocated page.
And we can't setup a new hypercall page by writing something different
to HV_X64_MSR_HYPERCALL, right?
>
> The suspend, resume and cleanup paths remain untouched because they are
> not supported in this setup yet.
>
> Signed-off-by: Lillian Grassin-Drake <ligrassi at microsoft.com>
> Signed-off-by: Sunil Muthuswamy <sunilmut at microsoft.com>
> Signed-off-by: Nuno Das Neves <nudasnev at microsoft.com>
> Co-Developed-by: Lillian Grassin-Drake <ligrassi at microsoft.com>
> Co-Developed-by: Sunil Muthuswamy <sunilmut at microsoft.com>
> Co-Developed-by: Nuno Das Neves <nudasnev at microsoft.com>
> Signed-off-by: Wei Liu <wei.liu at kernel.org>
> ---
>  arch/x86/hyperv/hv_init.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index 0eec1ed32023..26233aebc86c 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -25,6 +25,7 @@
>  #include <linux/cpuhotplug.h>
>  #include <linux/syscore_ops.h>
>  #include <clocksource/hyperv_timer.h>
> +#include <linux/highmem.h>
>  
>  /* Is Linux running as the root partition? */
>  bool hv_root_partition;
> @@ -448,8 +449,29 @@ void __init hyperv_init(void)
>  
>  	rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
>  	hypercall_msr.enable = 1;
> -	hypercall_msr.guest_physical_address = vmalloc_to_pfn(hv_hypercall_pg);
> -	wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> +
> +	if (hv_root_partition) {
> +		struct page *pg;
> +		void *src, *dst;
> +
> +		/*
> +		 * Order is important here. We must enable the hypercall page
> +		 * so it is populated with code, then copy the code to an
> +		 * executable page.
> +		 */
> +		wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> +
> +		pg = vmalloc_to_page(hv_hypercall_pg);
> +		dst = kmap(pg);
> +		src = memremap(hypercall_msr.guest_physical_address << PAGE_SHIFT,
PAGE_SIZE,
> +				MEMREMAP_WB);
memremap() can fail...
> +		memcpy(dst, src, PAGE_SIZE);
> +		memunmap(src);
> +		kunmap(pg);
> +	} else {
> +		hypercall_msr.guest_physical_address = vmalloc_to_pfn(hv_hypercall_pg);
> +		wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> +	}
Why can't we do wrmsrl() for both cases here?
>  
>  	/*
>  	 * Ignore any errors in setting up stimer clockevents
-- 
Vitaly