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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id m23sm8009179wmi.19.2020.09.15.04.02.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 15 Sep 2020 04:02:10 -0700 (PDT) From: Vitaly Kuznetsov To: Wei Liu Cc: Wei Liu , Linux on Hyper-V List , virtualization@lists.linux-foundation.org, Linux Kernel List , Michael Kelley , Vineeth Pillai , Sunil Muthuswamy , Nuno Das Neves , Lillian Grassin-Drake , "K. Y. Srinivasan" , Haiyang Zhang , Stephen Hemminger , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "maintainer\:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)" , "H. Peter Anvin" Subject: Re: [PATCH RFC v1 08/18] x86/hyperv: handling hypercall page setup for root In-Reply-To: <20200915103710.cqmdvzh5lys4wsqo@liuwe-devbox-debian-v2> References: <20200914112802.80611-1-wei.liu@kernel.org> <20200914112802.80611-9-wei.liu@kernel.org> <87v9gfjpoi.fsf@vitty.brq.redhat.com> <20200915103710.cqmdvzh5lys4wsqo@liuwe-devbox-debian-v2> Date: Tue, 15 Sep 2020 13:02:08 +0200 Message-ID: <87pn6njob3.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Wei Liu writes: > On Tue, Sep 15, 2020 at 12:32:29PM +0200, Vitaly Kuznetsov wrote: >> Wei Liu 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? >> > > My understanding is that we can't, but Sunil can maybe correct me. > >> > >> > The suspend, resume and cleanup paths remain untouched because they are >> > not supported in this setup yet. >> > >> > Signed-off-by: Lillian Grassin-Drake >> > Signed-off-by: Sunil Muthuswamy >> > Signed-off-by: Nuno Das Neves >> > Co-Developed-by: Lillian Grassin-Drake >> > Co-Developed-by: Sunil Muthuswamy >> > Co-Developed-by: Nuno Das Neves >> > Signed-off-by: Wei Liu >> > --- >> > 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 >> > #include >> > #include >> > +#include >> > >> > /* 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... > > And we don't care here, if it fails, we would rather it panic or oops. > > I was relying on the fact that copying from / to a NULL pointer will > cause the kernel to crash. But of course it wouldn't hurt to explicitly > panic here. > >> >> > + 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? >> > > Because the hypercall page has already been set up when Linux is the > root. But you already do wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64) in 'if (hv_root_partition)' case above, that's why I asked. > > I could've tried writing to the MSR again, but because the behaviour > here is not documented and subject to change so I didn't bother trying. > > Wei. > >> > >> > /* >> > * Ignore any errors in setting up stimer clockevents >> >> -- >> Vitaly >> > -- Vitaly