Re: [Xen-devel] [PATCH 9/9] x86: PCID is unused when !PV

From: "Roger Pau Monné" <roger.pau@citrix.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: George Dunlap <George.Dunlap@eu.citrix.com>,
	AndrewCooper <andrew.cooper3@citrix.com>, Wei Liu <wl@xen.org>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [Xen-devel] [PATCH 9/9] x86: PCID is unused when !PV
Date: Thu, 12 Sep 2019 17:57:43 +0200	[thread overview]
Message-ID: <20190912155743.jwb6gzqjfjrokh6h@Air-de-Roger> (raw)
In-Reply-To: <38a24f1a-9ed4-edb1-a0e2-193170fc6eee@suse.com>

On Thu, Sep 12, 2019 at 05:48:16PM +0200, Jan Beulich wrote:
> On 12.09.2019 17:31, Roger Pau Monné  wrote:
> > On Wed, Sep 11, 2019 at 05:26:46PM +0200, Jan Beulich wrote:
> >> @@ -301,8 +305,12 @@ static inline void write_cr4(unsigned lo
> >>  {
> >>      struct cpu_info *info = get_cpu_info();
> >>  
> >> +#ifdef CONFIG_PV
> >>      /* No global pages in case of PCIDs enabled! */
> >>      ASSERT(!(val & X86_CR4_PGE) || !(val & X86_CR4_PCIDE));
> >> +#else
> >> +    ASSERT(!(val & X86_CR4_PCIDE));
> > 
> > That assert seems quite pointless, you have set X86_CR4_PCIDE to 0, so
> > this is never going to trigger?
> 
> I was about to drop this, but I have to take back my earlier
> reply: The #ifdef you talk about is in flushtlb.c, whereas
> here we're in processor.h.

Oh yes, sorry for not realizing. In order to avoid the ifdefary maybe
you could write the above as:

ASSERT((IS_ENABLED(CONFIG_PV) && !(val & X86_CR4_PGE)) ||
       !(val & X86_CR4_PCIDE));

I don't have a strong opinion though, maybe my proposed version is
actually harder to read than the ifdef'ed one.

Thanks, Roger.

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