Re: x86 SGDT emulation for Wine

[Elizabeth Figura <zfigura@codeweavers.com>]

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:
> >
> >int get_vga_selector()
> >{
> >
> >    sgdt(&gdt_size, &gdt_ptr);
> >    sldt(&ldt_segment);
> >    ++gdt_size;
> >    descriptor = gdt_ptr;
> >    while (descriptor->base != 0xa0000)
> >    {
> >    
> >        ++descriptor;
> >        gdt_size -= sizeof(*descriptor);
> >        if (!gdt_size)
> >        
> >            break;
> >    
> >    }
> >    
> >    if (gdt_size)
> >    
> >        return (descriptor - gdt_ptr) << 3;
> >    
> >    descriptor = gdt_ptr[ldt_segment >> 3]->base;
> >    ldt_size = gdt_ptr[ldt_segment >> 3]->limit + 1;
> >    while (descriptor->base != 0xa0000)
> >    {
> >    
> >        ++descriptor;
> >        ldt_size -= sizeof(*descriptor);
> >        if (!ldt_size)
> >        
> >            break;
> >    
> >    }
> >    
> >    if (ldt_size)
> >    
> >        return (descriptor - ldt_ptr) << 3;
> >    
> >    return 0;
> >
> >}
> >
> >
> >Currently we emulate IDT access. On a read fault, we execute sidt
> >ourselves, check if the read address falls within the IDT, and return some
> >dummy data from the exception handler if it does [1]. We can easily enough
> >implement GDT access as well this way, and there is even an out-of-tree
> >patch written some years ago that does this, and helps the game run.
> >
> >However, there are two problems that I have observed or anticipated:
> >
> >(1) On systems with UMIP, the kernel emulates sgdt instructions and returns
> >a consistent address which we can guarantee is invalid. However, it also
> >returns a size of zero. The program doesn't expect this (cf. the way the
> >loop is written above) and I believe will effectively loop forever in that
> >case, or until it finds the VGA selector or hits invalid memory.
> >
> >    I see two obvious ways to fix this: either adjust the size of the fake
> >
> >kernel GDT, or provide a switch to stop emulating and let Wine handle it.
> >The latter may very well a more sustainable option in the long term
> >(although I'll admit I can't immediately come up with a reason why, other
> >than "we might need to raise the size yet again".)
> >
> >    Does anyone have opinions on this particular topic? I can look into
> >
> >writing a patch but I'm not sure what the best approach is.
> >
> >(2) On 64-bit systems without UMIP, sgdt returns a truncated address when
> >in 32-bit mode. This truncated address in practice might point anywhere in
> >the address space, including to valid memory.
> >
> >    In order to fix this, we would need the kernel to guarantee that the
> >    GDT
> >
> >base points to an address whose bottom 32 bits we can guarantee are
> >inaccessible. This is relatively easy to achieve ourselves by simply
> >mapping those pages as noaccess, but it also means that those pages can't
> >overlap something we need; we already go to pains to make sure that
> >certain parts of the address space are free. Broadly anything above the 2G
> >boundary *should* be okay though. Is this feasible?
> >
> >    We could also just decide we don't care about systems without UMIP, but
> >
> >that seems a bit unfortunate; it's not that old of a feature. But I also
> >have no idea how hard it would be to make this kind of a guarantee on the
> >kernel side.
> >
> >    This is also, theoretically, a problem for the IDT, except that on the
> >
> >machines I've tested, the IDT is always at 0xfffffe0000000000. That's not
> >great either (it's certainly caused some weirdness and confusion when
> >debugging, when we unexpectedly catch an unrelated null pointer access) but
> >it seems to work in practice.
> >
> >--Zeb
> >
> >[1] https://source.winehq.org/git/wine.git/blob/HEAD:/dlls/krnl386.exe16/
> >instr.c#l702
> 
> 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, 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. 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...)

  I'm concerned that this will be very expensive. Most VM users don't need to 
exit on every syscall. While I haven't tested KVM, I think some other Wine 
developers actually did a similar experiment using a hypervisor to solve some 
other problem (related to 32-bit support on Mac OS), and exiting the 
hypervisor was prohibitively slow.

  Alternatively we ship *more* components to reimplement these things inside 
the VM (e.g. virgl/venus for GPU hardware, other virtio bits for interacting 
with e.g. multimedia hardware? enough of a cache to make block I/O reasonably 
fast, a few layers of networking code...), which looks more and more ugly.

If nothing else, it's a huge hammer to fix this one problem for an application 
which doesn't even currently work in Wine, *and* which isn't even a problem on 
sufficiently new hardware (and to fix other GDT problems which are only 
theoretical at this point.)

--Zeb