linux-kernel - Re: [PATCH v5 1/3] drm/shmem: add support for per object caching flags.

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Message-ID: <20200227075321.ki74hfjpnsqv2yx2@sirius.home.kraxel.org>
Date:   Thu, 27 Feb 2020 08:53:21 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     dri-devel@...ts.freedesktop.org, Guillaume.Gardet@....com,
        David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org, gurchetansingh@...omium.org,
        tzimmermann@...e.de
Subject: Re: [PATCH v5 1/3] drm/shmem: add support for per object caching
 flags.

  Hi,

> > +		if (!shmem->map_cached)
> > +			prot = pgprot_writecombine(prot);
> >   		shmem->vaddr = vmap(shmem->pages, obj->size >> PAGE_SHIFT,
> > -				    VM_MAP, pgprot_writecombine(PAGE_KERNEL));
> > +				    VM_MAP, prot)
> 
> 
> Wouldn't a vmap with pgprot_writecombine() create conflicting mappings with
> the linear kernel map which is not write-combined?

I think so, yes.

> Or do you change the linear kernel map of the shmem pages somewhere?

Havn't seen anything doing so while browsing the code.

> vmap bypassess at least the
> x86 PAT core mapping consistency check and this could potentially cause
> spuriously overwritten memory.

Well, I don't think the linear kernel map is ever used to access the
shmem gem objects.  So while this isn't exactly clean it shouldn't
cause problems in practice.

Suggestions how to fix that?

The reason I need cachable gem object mappings for virtio-gpu is because
we have a inconsistency between host (cached) and guest (wc) otherwise.

> > +	}
> >   	if (!shmem->vaddr) {
> >   		DRM_DEBUG_KMS("Failed to vmap pages\n");
> > @@ -540,7 +545,9 @@ int drm_gem_shmem_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
> >   	}
> >   	vma->vm_flags |= VM_MIXEDMAP | VM_DONTEXPAND;
> > -	vma->vm_page_prot = pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
> > +	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
> > +	if (!shmem->map_cached)
> > +		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
> 
> Same thing here. Note that vmf_insert_page() which is used by the fault
> handler also bypasses the x86 PAT  consistency check, whereas
> vmf_insert_mixed() doesn't.

vmap + mmap are consistent though, so this likewise shouldn't cause
issues in practice.

> >   	vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
> 
> At least with SME or SEV encryption, where shmem memory has its kernel map
> set to encrypted, creating conflicting mappings is explicitly disallowed.
> BTW, why is mmap mapping decrypted while vmap isn't?

Ok, that sounds like a real problem.  Have to check.

cheers,
  Gerd

PS: Given we are discussing pre-existing issues in the code I think the
    series can be merged nevertheless.