Re: [QEMU TCG] Qeustion about the PCID Feature in TCG

[Paolo Bonzini]

On 18/02/21 12:43, Alex Bennée wrote:
> Kaifeng Xu <kaifengx@princeton.edu> writes:
>
> > Hi,
> > I am running QEMU in TCG mode (my server doesn't have kvm support), and I
> > am getting the memory traces in a x86 guest machine of all memory accesses,
> > including the PCID (process-context identifier, and I need that for my
> > current research), on a linux host. I have seen the TCG PCID feature flag
> > is commented out in target/i386/cpu.c, and if I directly emulate a CPU
> > which has PCID flag I got the warning:
> >
> > warning: TCG doesn't support requested feature: CPUID.01H:ECX.pcid [bit 17]
> >
> > so I uncommented the CPUID_EXT_PCID in TCG_EXT_FEATURES, and
> > the CPUID_7_0_EBX_INVPCID in TCG_7_0_EBX_FEATURES, in  target/i386/cpu.c. I
> > have seen the PCID in the cr3 register in cpu get set, my question is that
> > I am not sure if there will be any issues with this flag. Why is it
> > commented out by default?
>
> Typically the TCG implementation of x86 is fairly behind the features
> that may be reported or need to be handled for KVM. For example TCG
> doesn't support the most recent AVX instructions.
>
> I can't speak to how much effort it would be to implement PCID for x86
> TCG, that is probably a question best aimed at the maintainers for the
> architecture (cc'd).

Little, overall.  You don't have to _actually_ support PCID-tagging in 
the TLB, you only need to support the instruction; in general, using TCG 
as a benchmarking platform for something that depends on MMU events such 
as TLB misses is not a good idea).

In this case, everything seemingly works because QEMU is not checking 
reserved bits in CR3 and CR4 (see cpu_x86_update_cr3 and 
cpu_x86_update_cr4 in target/i386/helper.c).

A smaller issue is that PCID without the INVPCID instruction makes 
little sense, so you would have to implement that as well.

Paolo

> > Just in case, I will put my commands here. Here are my configurations for
> > compiling:
> >
> > *./configure --prefix=/tigress/kaifengx/sysroot/
> > --target-list=x86_64-softmmu,x86_64-linux-user --disable-kvm --disable-vnc
> > --enable-gtk --enable-plugins*
> >
> > And my command for running the QEMU:
> >
> >
> > *qemu-system-x86_64 \*
> > *    -cpu qemu64,+pcid\    -m 4G \    -icount shift=0 \    -drive
> > if=virtio,file=${DISK},cache=none \    -device pqii \    -trace
> > events=`pwd`/events \    -D ${LOG_FILE} \    -net
> > user,hostfwd=tcp::10022-:22 \    -net nic \    -display none \
> > -nographic \    -plugin QEMU_PATH/tests/plugin/libtlb.so,arg=inline,arg=io*
>
> Ahh good to see another plugin user. Be aware I just recently posted
> a pull request that fixes some recent breakages in the inline handling.
>
> What are you trying to measure with the plugin?