Re: [PATCH 03/22] x86/dom0: only disable SMAP for the PV dom0 build

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jul 29, 2024 at 04:52:22PM +0100, Andrew Cooper wrote:
> On 29/07/2024 12:53 pm, Jan Beulich wrote:
> > On 26.07.2024 17:21, Roger Pau Monne wrote:
> >> The PVH dom0 builder doesn't switch page tables and has no need to run with
> >> SMAP disabled.
> >>
> >> Put the SMAP disabling close to the code region where it's necessary, as it
> >> then becomes obvious why switch_cr3_cr4() is required instead of
> >> write_ptbase().
> >>
> >> Note removing SMAP from cr4_pv32_mask is not required, as we never jump 
> >> into
> >> guest context, and hence updating the value of cr4_pv32_mask is not 
> >> relevant.
> > I'm okay-ish with that being dropped, but iirc the goal was to keep the
> > variable in sync with CPU state.
> 
> Removing SMAP from cr4_pv32_mask is necessary.
> 
> Otherwise IST vectors will reactive SMAP behind the back of the dombuilder.
> 
> This will probably only manifest in practice in a CONFIG_PV32=y build,

Sorry, I'm possibly missing some context here.  When running the dom0
builder we switch to the guest page-tables, but not to the guest vCPU,
(iow: current == idle) and hence the context is always the Xen
context.

Why would the return path of the IST use cr4_pv32_mask when the
context in which the IST happened was the Xen one, and the current
vCPU is the idle one (a 64bit PV guest from Xen's PoV).

My understanding is that cr4_pv32_mask should only be used when the
current context is running a 32bit PV vCPU.

Thanks, Roger.