linux-kernel - Re: [PATCH v2 08/17] x86/hyperv: handling hypercall page setup for root

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Date:   Thu, 12 Nov 2020 16:51:09 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Wei Liu <wei.liu@...nel.org>,
        Linux on Hyper-V List <linux-hyperv@...r.kernel.org>
Cc:     virtualization@...ts.linux-foundation.org,
        Linux Kernel List <linux-kernel@...r.kernel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Sunil Muthuswamy <sunilmut@...rosoft.com>,
        Nuno Das Neves <nunodasneves@...ux.microsoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        Lillian Grassin-Drake <ligrassi@...rosoft.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 08/17] x86/hyperv: handling hypercall page setup for
 root

Wei Liu <wei.liu@...nel.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.
>
> The suspend, resume and cleanup paths remain untouched because they are
> not supported in this setup yet.

What about adding BUG_ONs there then?

>
> Signed-off-by: Lillian Grassin-Drake <ligrassi@...rosoft.com>
> Signed-off-by: Sunil Muthuswamy <sunilmut@...rosoft.com>
> Signed-off-by: Nuno Das Neves <nunodasneves@...ux.microsoft.com>
> Co-Developed-by: Lillian Grassin-Drake <ligrassi@...rosoft.com>
> Co-Developed-by: Sunil Muthuswamy <sunilmut@...rosoft.com>
> Co-Developed-by: Nuno Das Neves <nunodasneves@...ux.microsoft.com>
> Signed-off-by: Wei Liu <wei.liu@...nel.org>
> ---
>  arch/x86/hyperv/hv_init.c | 32 ++++++++++++++++++++++++++++++--
>  1 file changed, 30 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index 73b0fb851f76..9fcaf741be99 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;
> @@ -438,8 +439,35 @@ 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;
> +
> +		/*
> +		 * For the root partition, the hypervisor will set up its
> +		 * hypercall page. The hypervisor guarantees it will not show
> +		 * up in the root's address space. The root can't change the
> +		 * location of the hypercall page.
> +		 *
> +		 * 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);
> +		BUG_ON(!(src && dst));
> +		memcpy(dst, src, PAGE_SIZE);

Super-nit: while on x86 PAGE_SIZE always matches HV_HYP_PAGE_SIZE, would
it be more accurate to use the later here?

> +		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);
> +	}
>  
>  	/*
>  	 * Ignore any errors in setting up stimer clockevents

-- 
Vitaly