linux-kernel - Re: [PATCH] x86/hyperv: Use per cpu initial stack for vtl context

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Date: Thu, 1 Feb 2024 19:25:40 -0800
From: Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
To: Michael Kelley <mhklinux@...look.com>
Cc: "kys@...rosoft.com" <kys@...rosoft.com>,
	"haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
	"wei.liu@...nel.org" <wei.liu@...nel.org>,
	"decui@...rosoft.com" <decui@...rosoft.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"bp@...en8.de" <bp@...en8.de>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
	"dwmw@...zon.co.uk" <dwmw@...zon.co.uk>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ssengar@...rosoft.com" <ssengar@...rosoft.com>
Subject: Re: [PATCH] x86/hyperv: Use per cpu initial stack for vtl context

On Thu, Feb 01, 2024 at 07:53:59PM +0000, Michael Kelley wrote:
> From: Saurabh Sengar <ssengar@...ux.microsoft.com> Sent: Thursday, February 1, 2024 10:26 AM
> > 
> > Currently, the secondary vCPUs in Hyper-V VTL context lack support for
> > parallel startup. Therefore, relying on the single initial_stack fetched
> > from the current task structure suffices for all vCPUs.
> > 
> > However, common initial_stack risks stack corruption when parallel startup
> > is enabled. In order to facilitate parallel startup, use the initial_stack
> > from the per CPU idle thread instead of the current task.
> > 
> > Fixes: 18415f33e2ac ("cpu/hotplug: Allow "parallel" bringup up to CPUHP_BP_KICK_AP_STATE")
> > Signed-off-by: Saurabh Sengar <ssengar@...ux.microsoft.com>
> > ---
> >  arch/x86/hyperv/hv_vtl.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
> > index 96e6c51515f5..a54b46b673de 100644
> > --- a/arch/x86/hyperv/hv_vtl.c
> > +++ b/arch/x86/hyperv/hv_vtl.c
> > @@ -12,6 +12,7 @@
> >  #include <asm/i8259.h>
> >  #include <asm/mshyperv.h>
> >  #include <asm/realmode.h>
> > +#include <../kernel/smpboot.h>
> > 
> >  extern struct boot_params boot_params;
> >  static struct real_mode_header hv_vtl_real_mode_header;
> > @@ -71,7 +72,8 @@ static int hv_vtl_bringup_vcpu(u32 target_vp_index, u64 eip_ignored)
> >  	struct ldttss_desc *ldt;
> >  	struct desc_struct *gdt;
> > 
> > -	u64 rsp = current->thread.sp;
> > +	struct task_struct *idle = idle_thread_get(target_vp_index);
> 
> The "VP index" of a vCPU is a Hyper-V concept, and may not match
> the Linux concept of a CPU number.   In most cases, they *do* match,
> so your testing of this patch probably worked.  But there's no guarantee
> that they match.  The Hyper-V TLFS does not even guarantee that VP
> indices are dense or that they start with 0 (even though they do in
> current versions of Hyper-V).
> 
> As a different kind of example, in a kdump kernel, Linux labels the
> booting CPU as CPU 0, but it may not be the 0th CPU in the guest
> VM, and hence may not have VP index of 0.  Of course, in a kdump
> kernel nr_cpus is typically 1, so you aren't bringing up secondary
> CPUs.  But sometimes kdump kernels boot with nr_cpus=2 or greater,
> in which case the mismatch would occur.
> 
> This conceptual difference in VP index and Linux CPU numbers is why
> the hv_vp_index array exists -- to map from a Linux CPU number to a
> Hyper-V VP index, and thereby avoid assuming they are equal.
> 
> So before hv_vtl_wakeup_secondary_cpu() calls this function, it needs
> to separately map the apicid to a Linux CPU number, which can then
> be passed to idle_thread_get().
> 
> Michael

Thanks for the review. I will fix this in V2.

> 
> > +	u64 rsp = (unsigned long)idle->thread.sp;
> >  	u64 rip = (u64)&hv_vtl_ap_entry;
> > 
> >  	native_store_gdt(&gdt_ptr);
> > --
> > 2.34.1
> > 
>