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

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Date:   Thu, 20 Oct 2022 16:30:29 +0800
From:   Zhao Liu <zhao1.liu@...ux.intel.com>
To:     Ira Weiny <ira.weiny@...el.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 Tue, Oct 18, 2022 at 07:58:49PM -0700, Ira Weiny wrote:
> Date: Tue, 18 Oct 2022 19:58:49 -0700
> From: Ira Weiny <ira.weiny@...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>

Thanks! Let me quickly refresh a v2 patch without "Fixes" tag.

Zhao

> 
> > 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
> >