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id = 664
title = "hvf-accelerated x86_64 incorrectly reports virtual address bit width via CPUID"
state = "opened"
created_at = "2021-10-11T21:56:41.679Z"
closed_at = "n/a"
labels = ["accel: HVF", "target: i386"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/664"
host-os = "macOS 10.14.6"
host-arch = "x86_64"
qemu-version = "QEMU emulator version 6.0.0"
guest-os = "SerenityOS (https://github.com/SerenityOS/serenity/)"
guest-arch = "x86_64"
description = """When running qemu-system-x86_64 with hvf acceleration enabled the maximum extended cpuid function (available via EAX=0x80000000) is reported to be 0x80000001, which means that physical address and virtual address bit width (which is supposed to be reported via EAX=0x80000008) is not available. As per the intel IA32/64 manual: `Processors that do not support CPUID function 80000008H, support a linear-address width of 32.`, while in actuality qemu-system-x86_64 with hvf acceleration supports virtual addresses of up to 48 bit in width, like most modern x86_64 processors."""
reproduce = """This can be observed when running SerenityOS on x86_64 qemu with hvf acceleration based on the following dmesg lines:
```
[Kernel]: CPU[0]: Physical address bit width: 36
[Kernel]: CPU[0]: Virtual address bit width: 32
```
But can also be reproduced by running the CPUID instruction with EAX set to 0x80000000 and observing that the returned value is 0x80000001."""
additional = """The best way to resolve this as far as I can tell is to expose the 0x80000008 CPUID function and report the real values.
NOTE: This is a report of the underlying bug that was found during the investigation of an issue raised in the SerenityOS repository, see https://github.com/SerenityOS/serenity/issues/10382 for more information."""
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