linux-kernel - Re: [RFC v2 01/32] x86/paravirt: Introduce CONFIG_PARAVIRT

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Date:   Wed, 12 May 2021 15:07:32 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Juergen Gross <jgross@...e.com>
Cc:     Andi Kleen <ak@...ux.intel.com>, Borislav Petkov <bp@...en8.de>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 01/32] x86/paravirt: Introduce CONFIG_PARAVIRT_XL

On Mon, May 10, 2021 at 05:56:05PM +0200, Juergen Gross wrote:
> On 10.05.21 17:52, Andi Kleen wrote:
> > \
> > > > > CONFIG_PARAVIRT_XL will be used by TDX that needs couple of paravirt
> > > > > calls that were hidden under CONFIG_PARAVIRT_XXL, but the rest of the
> > > > > config would be a bloat for TDX.
> > > > 
> > > > Used how? Why is it bloat for TDX?
> > > 
> > > Is there any major downside to move the halt related pvops functions
> > > from CONFIG_PARAVIRT_XXL to CONFIG_PARAVIRT?
> > 
> > I think the main motivation is to get rid of all the page table related
> > hooks for modern configurations. These are the bulk of the annotations
> > and  cause bloat and worse code. Shadow page tables are really obscure
> > these days and very few people still need them and it's totally
> > reasonable to build even widely used distribution kernels without them.
> > On contrast most of the other hooks are comparatively few and also on
> > comparatively slow paths, so don't really matter too much.
> > 
> > I think it would be ok to have a CONFIG_PARAVIRT that does not have page
> > table support, and a separate config option for those (that could be
> > eventually deprecated).
> > 
> > But that would break existing .configs for those shadow stack users,
> > that's why I think Kirill did it the other way around.
> 
> No. We have PARAVIRT_XXL for Xen PV guests, and we have PARAVIRT for
> other hypervisor's guests, supporting basically the TLB flush operations
> and time related operations only. Adding the halt related operations to
> PARAVIRT wouldn't break anything.

Yeah, I think we can do this. It should be fine.

-- 
 Kirill A. Shutemov