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    SubjectRe: [PATCH] x86/hyperv: Suspend/resume the VP assist page for hibernation
    On Thu, Apr 16, 2020 at 11:29:59PM -0700, Dexuan Cui wrote:
    > Unlike the other CPUs, CPU0 is never offlined during hibernation. So in the
    > resume path, the "new" kernel's VP assist page is not suspended (i.e.
    > disabled), and later when we jump to the "old" kernel, the page is not
    > properly re-enabled for CPU0 with the allocated page from the old kernel.
    >
    > So far, the VP assist page is only used by hv_apic_eoi_write(). When the
    > page is not properly re-enabled, hvp->apic_assist is always 0, so the
    > HV_X64_MSR_EOI MSR is always written. This is not ideal with respect to
    > performance, but Hyper-V can still correctly handle this.
    >
    > The issue is: the hypervisor can corrupt the old kernel memory, and hence
    > sometimes cause unexpected behaviors, e.g. when the old kernel's non-boot
    > CPUs are being onlined in the resume path, the VM can hang or be killed
    > due to virtual triple fault.

    I don't quite follow here.

    The first sentence is rather alarming -- why would Hyper-V corrupt
    guest's memory (kernel or not)?

    Secondly, code below only specifies cpu0. What does it do with non-boot
    cpus on the resume path?

    Wei.

    >
    > Fix the issue by calling hv_cpu_die()/hv_cpu_init() in the syscore ops.
    >
    > Without the fix, hibernation can fail at a rate of 1/300 ~ 1/500.
    > With the fix, hibernation can pass a long-haul test of 2000 rounds.
    >
    > Fixes: 05bd330a7fd8 ("x86/hyperv: Suspend/resume the hypercall page for hibernation")
    > Cc: stable@vger.kernel.org
    > Signed-off-by: Dexuan Cui <decui@microsoft.com>
    > ---
    > arch/x86/hyperv/hv_init.c | 12 ++++++++++--
    > 1 file changed, 10 insertions(+), 2 deletions(-)
    >
    > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
    > index b0da5320bcff..4d3ce86331a3 100644
    > --- a/arch/x86/hyperv/hv_init.c
    > +++ b/arch/x86/hyperv/hv_init.c
    > @@ -72,7 +72,8 @@ static int hv_cpu_init(unsigned int cpu)
    > struct page *pg;
    >
    > input_arg = (void **)this_cpu_ptr(hyperv_pcpu_input_arg);
    > - pg = alloc_page(GFP_KERNEL);
    > + /* hv_cpu_init() can be called with IRQs disabled from hv_resume() */
    > + pg = alloc_page(GFP_ATOMIC);
    > if (unlikely(!pg))
    > return -ENOMEM;
    > *input_arg = page_address(pg);
    > @@ -253,6 +254,7 @@ static int __init hv_pci_init(void)
    > static int hv_suspend(void)
    > {
    > union hv_x64_msr_hypercall_contents hypercall_msr;
    > + int ret;
    >
    > /*
    > * Reset the hypercall page as it is going to be invalidated
    > @@ -269,12 +271,17 @@ static int hv_suspend(void)
    > hypercall_msr.enable = 0;
    > wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
    >
    > - return 0;
    > + ret = hv_cpu_die(0);
    > + return ret;
    > }
    >
    > static void hv_resume(void)
    > {
    > union hv_x64_msr_hypercall_contents hypercall_msr;
    > + int ret;
    > +
    > + ret = hv_cpu_init(0);
    > + WARN_ON(ret);
    >
    > /* Re-enable the hypercall page */
    > rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
    > @@ -287,6 +294,7 @@ static void hv_resume(void)
    > hv_hypercall_pg_saved = NULL;
    > }
    >
    > +/* Note: when the ops are called, only CPU0 is online and IRQs are disabled. */
    > static struct syscore_ops hv_syscore_ops = {
    > .suspend = hv_suspend,
    > .resume = hv_resume,
    > --
    > 2.19.1
    >

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