Re: x86 SGDT emulation for Wine

[Elizabeth Figura <zfigura@codeweavers.com>]

On Wednesday, January 3, 2024 9:33:10 AM CST H. Peter Anvin wrote:
> On January 3, 2024 7:19:02 AM PST, Sean Christopherson <seanjc@google.com> 
wrote:
> >On Tue, Jan 02, 2024, Elizabeth Figura wrote:
> >> On Wednesday, December 27, 2023 5:58:19 PM CST H. Peter Anvin wrote:
> >> > On December 27, 2023 2:20:37 PM PST, Elizabeth Figura 
<zfigura@codeweavers.com> wrote:
> >> > >Hello all,
> >> > >
> >> > >There is a Windows 98 program, a game called Nuclear Strike, which
> >> > >wants to
> >> > >do some amount of direct VGA access. Part of this is port I/O, which
> >> > >naturally throws SIGILL that we can trivially catch and emulate in
> >> > >Wine.
> >> > >The other part is direct access to the video memory at 0xa0000, which
> >> > >in
> >> > >general isn't a problem to catch and virtualize as well.
> >> > >
> >> > >However, this program is a bit creative about how it accesses that
> >> > >memory;
> >> > >instead of just writing to 0xa0000 directly, it looks up a segment
> >> > >descriptor whose base is at 0xa0000 and then uses the %es override to
> >
> >> > >write bytes. In pseudo-C, what it does is:
> >...
> >
> >> > A prctl() to set the UMIP-emulated return values or disable it (giving
> >> > SIGILL) would be easy enough.
> >> > 
> >> > For the non-UMIP case, and probably for a lot of other corner cases
> >> > like
> >> > relying on certain magic selector values and what not, the best option
> >> > really would be to wrap the code in a lightweight KVM container. I do
> >> > *not*
> >> > mean running the Qemu user space part of KVM; instead have Wine
> >> > interface
> >> > with /dev/kvm directly.
> >> > 
> >> > Non-KVM-capable hardware is basically historic at this point.
> >> 
> >> Sorry for the late response—I've been trying to do research on what would
> >> be necessary to use KVM (plus I made the poor choice of sending this
> >> during the holiday season...)
> >> 
> >> I'm concerned that KVM is going to be difficult or even intractable. Here
> >> are some of the problems that I (perhaps incorrectly) understand:
> >> 
> >> * As I am led to understand, there can only be one hypervisor on the
> >> machine at a time,
> >
> >No.  Only one instance of KVM-the-module is allowed, but there is no
> >arbitrary limit on the number of VMs that userspace can create.  The only
> >meaningful limitation is memory, and while struct kvm isn't tiny, it's not
> >_that_ big.>

Ah, thanks for the correction.

So if we're able to have one VM per thread, or one VM per process with one 
vcpu per thread (but that one is capped at 1024 at least right now?), and we 
don't risk running into any limits, that does make things a great deal easier.

Still, as Stefan said, I don't know if using a hypervisor is going to be 
plausible for speed reasons.

> >> and KVM has a hard limit on the number of vCPUs.
> >> 
> >>   The obvious way to use KVM for Wine is to make each (guest) thread a
> >>   vCPU.
> >> 
> >> That will, at the very least, run into the thread limit. In order to
> >> avoid
> >> that we'd need to ship a whole scheduler, which is concerning. That's a
> >> huge component to ship and a huge burden to keep updated. It also means
> >> we need to hoist *all* of the ipc and sync code into the guest, which
> >> will take an enormous amount of work.
> >> 
> >>   Moreover, because there can only be one hypervisor, and Wine is a
> >>   multi-
> >> 
> >> process beast, that means that we suddenly need to throw every process
> >> into
> >> the same VM.
> >
> >As above, this is wildly inaccurate.  The only KVM restriction with respect
> >to processes is that a VM is bound to the process (address space) that
> >created the VM.  There are no restrictions on the number of VMs that can
> >be created, e.g. a single process can create multiple VMs.
> >
> >> That has unfortunate implications regarding isolation (it's been a dream
> >> for years that we'd be able to share a single wine "VM" between multiple
> >> users), it complicates memory management (though perhaps not terribly?).
> >> And it means you can only have one Wine VM at a time, and can't use Wine
> >> at the same time as a "real" VM, neither of which are restrictions that
> >> currently exist.>> 
> >>   And it's not even like we can refactor—we'd have to rewrite tons of
> >>   code to
> >> 
> >> work inside a VM, but also keep the old code around for the cases where
> >> we
> >> don't have a VM and want to delegate scheduling to the host OS.
> >> 
> >> * Besides scheduling, we need to exit the VM every time we would normally
> >> call into Unix code, which in practice is every time that the
> >> application does an NT syscall, or uses a library which we delegate to
> >> the host (including e.g. GPU, multimedia, audio...)
> >
> >Maybe I misinterpreted Peter's suggestion, but at least in my mind I wasn't
> >thinking that the entire Wine process would run in a VM, but rather Wine
> >would run just the "problematic" code in a VM.
> 
> Yes, the idea would be that you would run the "problematic" code inside a VM
> *mapped 1:1 with the external address space*, i.e. use KVM simply as a
> special execution mode to give you more control of the fine grained machine
> state like the GDT. The code that you don't want executed in the VM context
> simply leave unmapped in the VM page tables and set up #PF to always exit
> the VM context.

So yes, as long as we *can* organize things such that we exit the hypervisor 
every time we want to call into Unix code, then that's feasible. We have a 
well-defined break between Windows and Unix code and it wouldn't be 
inordinately difficult to shove the VM exit into that break. My concern was 
that limitations on the number of VMs or vCPUs we can create would prevent us 
from doing that, and effectively require us to implement a lot more inside the 
VM, but as I understand that's not actually a problem.

That still leaves the question of performance though. If having to exit the VM 
that often for performance reasons isn't feasible, then that's still going to 
force us to implement from scratch an inordinate amount of kernel/library code 
inside the VM just to avoid the transition. Or, more likely, conclude that a 
hypervisor just isn't going to work for us.


I'm not at all familiar with the arch code, and I'm sure I'm not asking 
anything interesting, but is it really impossible to put CPU_ENTRY_AREA_RO_IDT 
somewhere that doesn't truncate to NULL, and to put the GDT at a fixed address 
as well?