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Date: Thu, 22 Jul 2021 21:45:36 +0530
From: Praveen Kumar <kumarpraveen@...ux.microsoft.com>
To: Wei Liu <wei.liu@...nel.org>
Cc: linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com,
bp@...en8.de, x86@...nel.org, hpa@...or.com,
viremana@...ux.microsoft.com, sunilmut@...rosoft.com,
nunodasneves@...ux.microsoft.com
Subject: Re: [PATCH v2] hyperv: root partition faults writing to VP ASSIST MSR
PAGE
On 22-07-2021 15:57, Wei Liu wrote:
> On Wed, Jul 21, 2021 at 11:33:02PM +0530, Praveen Kumar wrote:
>> 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
>> 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;
>> +
>> + 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]);
>
> I think about this a bit more, the NULL check for *hvp in hv_cpu_init in
> the original code is perhaps due to the code has opted to not free the
> page when disabling the VP assist page. When the CPU is brought back
> online, it does not want to allocate another page, but to use the one
> that's already allocated.
>
> So, since you listened to my suggestion to add a similar check, you need
> to reset hv_vp_assist_page to NULL here. Alternatively the check for
> *hvp can be dropped for the root path. Either way, the difference
> between root and non-root should be documented.
>
I would make it as NULL post memunmap as you suggested, so that we don't end up reusing the old/cached value. Before doing that, is there any use-case where hypervisor can allocate or change the VP assist page that can impact root kernel execution ?
> Wei.
>
>> + else
>> + wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, 0);
>> + }
>>
>> if (hv_reenlightenment_cb == NULL)
>> return 0;
>> --
>> 2.25.1
>>
Regards,
~Praveen.
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