On 06/13/18 21:03, Laszlo Ersek wrote: > On 06/13/18 20:20, Laszlo Ersek wrote: > >> * testing on aarch64/KVM: >> >> The graphics output looks great as long as I'm in the UEFI shell / the >> setup TUI / the grub menu. However, once I boot a Fedora 28 Server >> ISO, and the graphical Anacona Welcome screen appears, I get the exact >> same display corruption as before, with QemuVideoDxe + the VGA device >> model. I don't have the slightest idea why that is the case, but it's >> very visible, if I quickly move the thick blue line cursor around the >> language and keyboard layout selection lists. It's visible to the >> naked eye how dirty memory is gradually flushed to the display.
Small (or not so small) technical correction: the problem is that the guest OS writes directly to DRAM, but QEMU reads from the CPU cache. So I guess the gradual process that is visible to the naked eye is not "flushing" but "invalidation"; i.e. as the CPU caches become invalid, QEMU is gradually forced to reach out to DRAM and finally see what the guest OS put there. I guess. More below: > > Wait, I see "efifb" has a parameter called "nowc", and it disables > write-combining, according to "Documentation/fb/efifb.txt". > > Looking at the source code ("drivers/video/fbdev/efifb.c"), "nowc" > decides between: > >> if (nowc) >> info->screen_base = ioremap(efifb_fix.smem_start, >> efifb_fix.smem_len); >> else >> info->screen_base = ioremap_wc(efifb_fix.smem_start, >> efifb_fix.smem_len); > > Am I right to think that *both* of these ioremap() variants map the > phsyical address range as uncache-able? ("nowc" defaults to "false", and > I didn't specify "nowc" at all on the kernel command line.) > > Quoting "arch/arm/include/asm/io.h": > >> * Function Memory type Cacheability Cache hint >> * ioremap() Device n/a n/a >> * ioremap_nocache() Device n/a n/a >> * ioremap_cache() Normal Writeback Read allocate >> * ioremap_wc() Normal Non-cacheable n/a >> * ioremap_wt() Normal Non-cacheable n/a > > ioremap() implies "device memory" (by definition uncacheable only), > while ioremap_wc() is normal memory, but UC. Sigh. > > The include file "arch/arm64/include/asm/io.h" doesn't have such helpful > comments, but it does declare ioremap_cache() at least: > >> extern void __iomem *ioremap_cache(phys_addr_t phys_addr, size_t size); > > Now, I *guess* if I rebuilt the efifb driver to use ioremap_cache() -- > dependent on a new module parameter or some such --, the Linux guest > would start working as expected. Unfortunately, the Linux guest is > already pretty happy with virtio-gpu-pci; the question is how the > Windows guest would map the EFI framebuffer! > > Unfortunately, I cannot test ARM64 Windows guests ATM. > > So... If the consensus is that the edk2 code simply cannot get better > than this, and everything else is up to the guest OS(es), then I'm 100% > willing to push this version (with my minimal updates squashed). I've just remembered that Drew drew our attention earlier to: [PATCH 0/4] KVM/arm64: Cache maintenance relaxations http://mid.mail-archive.com/20180517103548.5622-1-marc.zyngier@arm.com I believe this means that, on an ARMv8.4 host, the ramfb device model will automatically work, in all guest OSes. If that's the case, I suggest we merge this series. Thanks Laszlo _______________________________________________ edk2-devel mailing list edk2-devel@lists.01.org https://lists.01.org/mailman/listinfo/edk2-devel