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Date: Thu, 22 Jul 2021 05:53:17 +0000
From: Michael Kelley <mikelley@...rosoft.com>
To: Praveen Kumar <kumarpraveen@...ux.microsoft.com>,
"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC: KY Srinivasan <kys@...rosoft.com>,
Haiyang Zhang <haiyangz@...rosoft.com>,
Stephen Hemminger <sthemmin@...rosoft.com>,
"wei.liu@...nel.org" <wei.liu@...nel.org>,
Dexuan Cui <decui@...rosoft.com>,
"tglx@...utronix.de" <tglx@...utronix.de>,
"mingo@...hat.com" <mingo@...hat.com>,
"bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
"hpa@...or.com" <hpa@...or.com>,
"viremana@...ux.microsoft.com" <viremana@...ux.microsoft.com>,
Sunil Muthuswamy <sunilmut@...rosoft.com>,
"nunodasneves@...ux.microsoft.com" <nunodasneves@...ux.microsoft.com>
Subject: RE: [PATCH v2] hyperv: root partition faults writing to VP ASSIST MSR
PAGE
From: Praveen Kumar <kumarpraveen@...ux.microsoft.com> Sent: Wednesday, July 21, 2021 11:03 AM
>
> For Root partition the VP assist pages are pre-determined by the
> hypervisor. The Root kernel is not allowed to change them to
> different locations. And thus, we are getting below stack as in
> current implementation Root is trying to perform write to specific
> MSR.
>
> [ 2.778197] unchecked MSR access error: WRMSR to 0x40000073 (tried to
> write 0x0000000145ac5001) at rIP: 0xffffffff810c1084
> (native_write_msr+0x4/0x30)
> [ 2.784867] Call Trace:
> [ 2.791507] hv_cpu_init+0xf1/0x1c0
> [ 2.798144] ? hyperv_report_panic+0xd0/0xd0
> [ 2.804806] cpuhp_invoke_callback+0x11a/0x440
> [ 2.811465] ? hv_resume+0x90/0x90
> [ 2.818137] cpuhp_issue_call+0x126/0x130
> [ 2.824782] __cpuhp_setup_state_cpuslocked+0x102/0x2b0
> [ 2.831427] ? hyperv_report_panic+0xd0/0xd0
> [ 2.838075] ? hyperv_report_panic+0xd0/0xd0
> [ 2.844723] ? hv_resume+0x90/0x90
> [ 2.851375] __cpuhp_setup_state+0x3d/0x90
> [ 2.858030] hyperv_init+0x14e/0x410
> [ 2.864689] ? enable_IR_x2apic+0x190/0x1a0
> [ 2.871349] apic_intr_mode_init+0x8b/0x100
> [ 2.878017] x86_late_time_init+0x20/0x30
> [ 2.884675] start_kernel+0x459/0x4fb
> [ 2.891329] secondary_startup_64_no_verify+0xb0/0xbb
>
> Since, the hypervisor already provides the VP assist page for root
> partition, we need to memremaps the memory from hypervisor for root
s/memremaps/memremap/
> kernel to use. The mapping is done in hv_cpu_init during bringup and
> is unmaped in hv_cpu_die during teardown.
>
> Signed-off-by: Praveen Kumar <kumarpraveen@...ux.microsoft.com>
> ---
> arch/x86/hyperv/hv_init.c | 53 ++++++++++++++++++++++++++-------------
> 1 file changed, 36 insertions(+), 17 deletions(-)
>
> changelog:
> v1: initial patch
> v2: commit message changes, removal of HV_MSR_APIC_ACCESS_AVAILABLE
> check and addition of null check before reading the VP assist MSR
> for root partition
>
> ---
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index 6f247e7e07eb..ffd3d3b37235 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -55,26 +55,41 @@ static int hv_cpu_init(unsigned int cpu)
> return 0;
>
> /*
> - * The VP ASSIST PAGE is an "overlay" page (see Hyper-V TLFS's Section
> - * 5.2.1 "GPA Overlay Pages"). Here it must be zeroed out to make sure
> - * we always write the EOI MSR in hv_apic_eoi_write() *after* the
> - * EOI optimization is disabled in hv_cpu_die(), otherwise a CPU may
> - * not be stopped in the case of CPU offlining and the VM will hang.
> + * For Root partition we need to map the hypervisor VP ASSIST PAGE
> + * instead of allocating a new page.
> */
> - if (!*hvp) {
> - *hvp = __vmalloc(PAGE_SIZE, GFP_KERNEL | __GFP_ZERO);
> - }
> + if (hv_root_partition) {
> + union hv_x64_msr_hypercall_contents hypercall_msr;
This isn't the correct variable type to be using here. Union
hv_x64_msr_hypercall_contents is specifically for HV_X64_MSR_HYPERCALL.
It also happens to be correct for HV_X64_MSR_VP_ASSIST_PAGE, but the
layout of the two MSRs could diverge in the future. Instead of using this union,
I would suggest just reading into a u64, and then mask as needed. The code in
the non-root-partition branch of the 'if' statement is similarly open coding
the needed shifting/masking to construct the value to write.
Or you could define another union specifically for the VP Assist page MSR.
I'm OK with either approach.
Michael
> +
> + rdmsrl(HV_X64_MSR_VP_ASSIST_PAGE, hypercall_msr.as_uint64);
> + /* remapping to root partition address space */
> + if (!*hvp)
> + *hvp = memremap(hypercall_msr.guest_physical_address <<
> + HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT,
> + PAGE_SIZE, MEMREMAP_WB);
> + WARN_ON(!(*hvp));
> + } else {
> + /*
> + * The VP ASSIST PAGE is an "overlay" page (see Hyper-V TLFS's
> + * Section 5.2.1 "GPA Overlay Pages"). Here it must be zeroed
> + * out to make sure we always write the EOI MSR in
> + * hv_apic_eoi_write() *after* theEOI optimization is disabled
> + * in hv_cpu_die(), otherwise a CPU may not be stopped in the
> + * case of CPU offlining and the VM will hang.
> + */
> + if (!*hvp)
> + *hvp = __vmalloc(PAGE_SIZE, GFP_KERNEL | __GFP_ZERO);
>
> - if (*hvp) {
> - u64 val;
> + if (*hvp) {
> + u64 val;
>
> - val = vmalloc_to_pfn(*hvp);
> - val = (val << HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT) |
> - HV_X64_MSR_VP_ASSIST_PAGE_ENABLE;
> + val = vmalloc_to_pfn(*hvp);
> + val = (val << HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT) |
> + HV_X64_MSR_VP_ASSIST_PAGE_ENABLE;
>
> - wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, val);
> + wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, val);
> + }
> }
> -
> return 0;
> }
>
> @@ -170,8 +185,12 @@ static int hv_cpu_die(unsigned int cpu)
>
> hv_common_cpu_die(cpu);
>
> - if (hv_vp_assist_page && hv_vp_assist_page[cpu])
> - wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, 0);
> + if (hv_vp_assist_page && hv_vp_assist_page[cpu]) {
> + if (hv_root_partition)
> + memunmap(hv_vp_assist_page[cpu]);
> + else
> + wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, 0);
> + }
>
> if (hv_reenlightenment_cb == NULL)
> return 0;
> --
> 2.25.1
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