linux-kernel - Re: [PATCH] x86/hyperv: Remove BUG_ON() for kmap_local

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Message-ID: <Y09n6dRN+zxsaLW/@iweiny-desk3>
Date:   Tue, 18 Oct 2022 19:58:49 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Zhao Liu <zhao1.liu@...ux.intel.com>
CC:     "K . Y . Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, <linux-hyperv@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        "Fabio M . De Francesco" <fmdefrancesco@...il.com>,
        Zhenyu Wang <zhenyu.z.wang@...el.com>,
        Zhao Liu <zhao1.liu@...el.com>
Subject: Re: [PATCH] x86/hyperv: Remove BUG_ON() for kmap_local_page()

On Wed, Oct 19, 2022 at 12:21:17AM +0800, Zhao Liu wrote:
> From: Zhao Liu <zhao1.liu@...el.com>
> 
> The commit 154fb14df7a3c ("x86/hyperv: Replace kmap() with
> kmap_local_page()") keeps the BUG_ON() to check if kmap_local_page()
> fails.
> 
> But in fact, kmap_local_page() always returns a valid kernel address
> and won't return NULL here. It will BUG on its own if it fails. [1]
> 
> So directly use memcpy_to_page() which creates local mapping to copy.
> 
> [1]: https://lore.kernel.org/lkml/YztFEyUA48et0yTt@iweiny-mobl/
> 
> Fixes: 154fb14df7a3 ("x86/hyperv: Replace kmap() with kmap_local_page()")

I don't know that a fixes is required here.  We are not looking to backport any
of this and the other patch was correct.  This is just a follow on cleanup.

> Suggested-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>

Code looks good.  Without the fixes.

Reviewed-by: Ira Weiny <ira.weiny@...el.com>

> Signed-off-by: Zhao Liu <zhao1.liu@...el.com>
> ---
>  arch/x86/hyperv/hv_init.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index 29774126e931..f66c5709324f 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -459,13 +459,11 @@ void __init hyperv_init(void)
>  		wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
>  
>  		pg = vmalloc_to_page(hv_hypercall_pg);
> -		dst = kmap_local_page(pg);
>  		src = memremap(hypercall_msr.guest_physical_address << PAGE_SHIFT, PAGE_SIZE,
>  				MEMREMAP_WB);
> -		BUG_ON(!(src && dst));
> -		memcpy(dst, src, HV_HYP_PAGE_SIZE);
> +		BUG_ON(!src);
> +		memcpy_to_page(pg, 0, src, HV_HYP_PAGE_SIZE);
>  		memunmap(src);
> -		kunmap_local(dst);
>  	} else {
>  		hypercall_msr.guest_physical_address = vmalloc_to_pfn(hv_hypercall_pg);
>  		wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> -- 
> 2.34.1
>