Re: [PATCH 1/1] x86/xen: Reset VCPU0 info pointer after shared_info remap

From: van der Linden, Frank
Date: Mon May 07 2018 - 14:00:21 EST


Hi Boris,

Thanks for the feedback.

ïOn 5/7/18, 8:13 AM, "Boris Ostrovsky" <boris.ostrovsky@xxxxxxxxxx> wrote:

> diff --git a/arch/x86/xen/enlighten_hvm.c b/arch/x86/xen/enlighten_hvm.c
> index 6b424da1ce75..c78b3e8fb2e5 100644
> --- a/arch/x86/xen/enlighten_hvm.c
> +++ b/arch/x86/xen/enlighten_hvm.c
> @@ -71,6 +71,19 @@ static void __init xen_hvm_init_mem_mapping(void)
> {
> early_memunmap(HYPERVISOR_shared_info, PAGE_SIZE);
> HYPERVISOR_shared_info = __va(PFN_PHYS(shared_info_pfn));
> +
> + /*
> + * The virtual address of the shared_info page has changed, so
> + * the vcpu_info pointer for VCPU 0 is now stale.

Is it "has changed" or "has changed if kaslr is on"?

It's "has changed". See commit 4ca83dcf4e3bc0c98836dbb97553792ca7ea5429 . It's a way to make kaslr work, but it's done regardless of whether it's enabled or not.

> + *
> + * The prepare_boot_cpu callback will re-initialize it via
> + * xen_vcpu_setup, but we can't rely on that to be called for
> + * old Xen versions (xen_have_vector_callback == 0).
> + *
> + * It is, in any case, bad to have a stale vcpu_info pointer
> + * so reset it now.
> + */
> + xen_vcpu_info_reset(0);


Why not xen_vcpu_setup(0)?

Basically, I wanted to be minimally invasive. xen_vcpu_setup does a little more work (tries to do the VCPU placement hypercall), and will be called later in any case. So doing just the basic xen_vcpu_info_reset for VCPU 0 seems like the best way to do it; it just re-iterates what is done for VCPU 0 earlier in boot, which is also a vcpu_info_reset.

Frank