Re: [PATCH v3] hyperv: root partition faults writing to VP ASSIST MSR PAGE

[Praveen Kumar <kumarpraveen@linux.microsoft.com>]

On 27-07-2021 22:15, Sunil Muthuswamy wrote:
> From: Praveen Kumar <kumarpraveen@linux.microsoft.com> Sent: Tuesday, July 27, 2021 3:41 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 memremap 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@linux.microsoft.com>
>> ---
>>  arch/x86/hyperv/hv_init.c          | 61 +++++++++++++++++++++---------
>>  arch/x86/include/asm/hyperv-tlfs.h |  9 +++++
>>  2 files changed, 53 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
>> v3: added new data structure to handle VP ASSIST MSR page and done
>>     handling in hv_cpu_init and hv_cpu_die
>>
>> ---
>> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
>> index 6f247e7e07eb..b859e42b4943 100644
>> --- a/arch/x86/hyperv/hv_init.c
>> +++ b/arch/x86/hyperv/hv_init.c
>> @@ -44,6 +44,7 @@ EXPORT_SYMBOL_GPL(hv_vp_assist_page);
>>
>>  static int hv_cpu_init(unsigned int cpu)
>>  {
>> +	union hv_vp_assist_msr_contents msr;
>>  	struct hv_vp_assist_page **hvp = &hv_vp_assist_page[smp_processor_id()];
>>  	int ret;
>>
>> @@ -54,27 +55,41 @@ static int hv_cpu_init(unsigned int cpu)
>>  	if (!hv_vp_assist_page)
>>  		return 0;
> 
> Not related to this code, but I am not sure about the usefulness of this NULL check as
> we have already accessed this pointer above. If it was NULL, things would already
> blow up.
> 
What I understood, hvp will point to "hv_vp_assist_page stack address + smp_processor_id()"
So, we are good, and this NULL check is required, as in when we de-reference the location, later in the code, it may fault.
Please do correct me if my understanding is wrong here. Thanks.

>>
>> -	/*
>> -	 * 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.
>> -	 */
>> -	if (!*hvp) {
>> -		*hvp = __vmalloc(PAGE_SIZE, GFP_KERNEL | __GFP_ZERO);
>> +	if (hv_root_partition) {
>> +		/*
>> +		 * For Root partition we get the hypervisor provided VP ASSIST
>> +		 * PAGE, instead of allocating a new page.
>> +		 */
>> +		rdmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
>> +
>> +		/* remapping to root partition address space */
> 
> Better to leave out comments that are obvious from the code.
> 

Sure will remove the comment here. Thanks.

>> +		if (!*hvp)
>> +			*hvp = memremap(msr.guest_physical_address <<
>> +					HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT,
>> +					PAGE_SIZE, MEMREMAP_WB);
>> +	} 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;
>> +	WARN_ON(!(*hvp));
>>
>> -		val = vmalloc_to_pfn(*hvp);
>> -		val = (val << HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT) |
>> -			HV_X64_MSR_VP_ASSIST_PAGE_ENABLE;
>> +	if (*hvp) {
>> +		if (!hv_root_partition)
>> +			msr.guest_physical_address = vmalloc_to_pfn(*hvp);
> 
> It's better to move this above in the else section where we are 'vmalloc' the page.
> If you just check for the NULL for the page above and return if NULL, that should
> clean up the code as well.
> 
>>

Ok, I was trying to make code cleaner, but I would have done better.

>> -		wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, val);
>> +		msr.enable = 1;
> 
> We should also set the reserved bits to 0.
> 
>> +		wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
>>  	}
>> -
>>  	return 0;
>>  }
>>
>> @@ -170,9 +185,21 @@ static int hv_cpu_die(unsigned int cpu)
>>
>>  	hv_common_cpu_die(cpu);
>>
>> -	if (hv_vp_assist_page && hv_vp_assist_page[cpu])
>> +	if (hv_vp_assist_page && hv_vp_assist_page[cpu]) {
>>  		wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, 0);
> 
> Its better to read the MSR, set the enable bit to 0 and write it back.
> 

Ok. probably this is the same code, which is being done in hv_cpu_init just with enable bit set to 0.
Let me try of a API / macro which can be used.

>>
>> +		if (hv_root_partition) {
>> +			/*
>> +			 * For Root partition the VP ASSIST page is mapped to
>> +			 * hypervisor provided page, and thus, we unmap the
>> +			 * page here and nullify it, so that in future we have
>> +			 * correct page address mapped in hv_cpu_init
>> +			 */
>> +			memunmap(hv_vp_assist_page[cpu]);
>> +			hv_vp_assist_page[cpu] = NULL;
>> +		}
> 
> For the guest case, where are we freeing the page?
> 

Yes, this I have observed,and looks like we don't want to allocate the page every-time, instead reuse the same.
If someone in the list can please provide some information or history behind this, it would give some insights. But for now,
I am leaving it as it is.

>> +	}
>> +
>>  	if (hv_reenlightenment_cb == NULL)
>>  		return 0;
>>
>> diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
>> index f1366ce609e3..2e4e87046aa7 100644
>> --- a/arch/x86/include/asm/hyperv-tlfs.h
>> +++ b/arch/x86/include/asm/hyperv-tlfs.h
>> @@ -288,6 +288,15 @@ union hv_x64_msr_hypercall_contents {
>>  	} __packed;
>>  };
>>
>> +union hv_vp_assist_msr_contents {
>> +	u64 as_uint64;
>> +	struct {
>> +		u64 enable:1;
>> +		u64 reserved:11;
>> +		u64 guest_physical_address:52;
> 
> 
> This field contains the page frame number and not the physical address. It is also
> better to drop the phrase 'guest' from the name as this applies to root as well.
> 

Ack.

> - Sunil


Regards,

~Praveen.
>