Is it normal that Write-Combining has no effect on QEMU

[devc1]

I recently implemented a full Page-Attribute-Table (PAT), With WB as default, and UC, WC, WT, WP... When I activate write combining on the Frame Buffer it doesn't have any effect on QEMU. But it has an 1000x Effect on Real Hardware. I am just curious, is this normal ??

[iansjack]

Are you using HAXM? https://www.qemu.org/2017/11/22/haxm-usage-windows/

[devc1]

No, My hardware doesn't support HAXM (I only need VMX). I tried Windows on QEMU and since it is well known that it has Write-Combining. It is seen that Graphics in it are veeryyy slow like mines without WC. Is this right ?? QEMU doesn't show any effects with WC ?

[Octocontrabass]

QEMU's emulated framebuffer is in ordinary RAM. Ordinary RAM does not benefit from write-combining the same way a framebuffer attached to the PCI(e) bus would.

[devc1]

I agree. So With SSE in QEMU lets say I can write 1 GB in a second. To fill 0-255 colors in a 1920p frame buffer it took almost 2.5 seconds. So, 1920*1080*4 is 8MB then to fill from black to white (255 white shades) you will need 2gb. Considering the latency that QEMU can take to show each of these pixel we can state that we're actually writing to RAM.

This maybe true, or QEMU is just putting on some latency with a Sleep() or a timer.