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Message-ID: <MW4PR21MB20021A51BE9960E10C08FE6FC0E99@MW4PR21MB2002.namprd21.prod.outlook.com>
Date: Tue, 27 Jul 2021 16:45:24 +0000
From: Sunil Muthuswamy <sunilmut@...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>,
"nunodasneves@...ux.microsoft.com" <nunodasneves@...ux.microsoft.com>
Subject: RE: [PATCH v3] hyperv: root partition faults writing to VP ASSIST MSR
PAGE
From: Praveen Kumar <kumarpraveen@...ux.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@...ux.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.
>
> - /*
> - * 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.
> + 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.
>
> - 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.
>
> + 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?
> + }
> +
> 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.
- Sunil
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