On Wed, Nov 18, 2020 at 12:20:11AM +0000, Michael Kelley wrote:
From: Dexuan Cui decui@microsoft.com Sent: Tuesday, November 17, 2020 4:03 PM
x86 Hyper-V used to essentially always overwrite the effective cache type of guest memory accesses to WB. This was problematic in cases where there is a physical device assigned to the VM, since that often requires that the VM should have control over cache types. Thus, on newer Hyper-V since 2018, Hyper-V always honors the VM's cache type, but unexpectedly Linux VM users start to complain that Linux VM's VRAM becomes very slow, and it turns out that Linux VM should not map the VRAM uncacheable by ioremap(). Fix this slowness issue by using ioremap_cache().
On ARM64, ioremap_cache() is also required as the host also maps the VRAM cacheable, otherwise VM Connect can't display properly with ioremap() or ioremap_wc().
With this change, the VRAM on new Hyper-V is as fast as regular RAM, so it's no longer necessary to use the hacks we added to mitigate the slowness, i.e. we no longer need to allocate physical memory and use it to back up the VRAM in Generation-1 VM, and we also no longer need to allocate physical memory to back up the framebuffer in a Generation-2 VM and copy the framebuffer to the real VRAM. A further big change will address these for v5.11.
Fixes: 68a2d20b79b1 ("drivers/video: add Hyper-V Synthetic Video Frame Buffer Driver") Tested-by: Boqun Feng boqun.feng@gmail.com Signed-off-by: Dexuan Cui decui@microsoft.com
Reviewed-by: Michael Kelley mikelley@microsoft.com
Applied to hyperv-fixes. Thanks.