On 10/29/2012 03:44 PM, Jan Kiszka wrote: > On 2012-10-29 08:09, Xiao Guangrong wrote: >> Jan, >> >> On 10/26/2012 06:35 PM, Jan Kiszka wrote: >> >>> This has two problems: We know it breaks at least Win 95 that overwrites >>> its F-segment during boot. And it applies changes to the shadowed area >>> (below 1 MB) also to the ROM area - I don't think that is the original >>> behaviour on real hardware. >> >> So what is the problem? It can break Win95's running? >> >> I tried to install win95 guest but it failed to boot regardless my patchset >> was applied or not. I found the information that win 95 is not supported at >> http://www.linux-kvm.org/page/Guest_Support_Status >> >> Note: before my patchset, Win 95 still can happily something into ROM area >> because readonly memory is actually writable on KVM. And win95 can not run >> on isapc with --no-kvm since it is no way to enable shadow ROM. > > Your patches causes regressions on TCG mode as that is perfectly fine > with booting Win95 so far. Aha, i tried accel=tcg, before my patchset, it works for -machine pc but failed for -machine isapc (known issue for seabios). After my patchset, it works fine for both -machine pc and isapc. :) > >> >>> >>> What we need is paravirtual shadow write control for the ISA PC. It's on >>> my todo list, maybe I will be able to look into this during the next week. >>> >> >> You idea is that modify the code of seabios and use a special way (PV) to >> notify Qemu to make the bios writable? > > Yes. > >> >> Actually, I am confused why the guest (including bios) persistently uses >> shadow ROM even if it is not supported (on ISA PC), i think the right way >> is move itself to RAM under this case, no? > > I've been told that Seabios has been built around that assumption and > the PV shadow control would be simpler to realize. Sounds the PV is complexer that directly making the bios area writable (if it works). > >> >>> BTW, your patch series should allow to drop the KVM special case from >>> pc_system_firmware_init. That version, btw, treats high and low BIOS >>> areas separately - but only reloads the upper area. Hmm... >>> >> >> You mean that also allow Qemu to use pflash to load bios if kvm is enabled? > > Yes. > >> We can not do that for pflash is a RD device which can not be directly written, >> kvm can not emulate the instruction which implicitly write the memory. (e.g: >> using this area as stack). > > Isn't enabling ROMD support for KVM that whole point of your patches? I It can generate MMIO exit if ROMD be written, that means the instruction needs kvm's help to be finished if it explicitly/implicitly write the memory. > do not see yet what prevents this still, but it should be fixed first. For the explicitly write memory access, it is easy to be fixed - we just need to fetch the instruction from EIP and emulate it. But for the implicitly memory access, fixing its emulation is really hard work. Really worth doing it? -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
